Final Exam Study Guide (Geoscience 331)
Final Exam Study Guide (Geoscience 331) Geoscience 331
Popular in Gems: The Science Behind the Sparkle
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This 54 page Study Guide was uploaded by Hannah James on Saturday December 5, 2015. The Study Guide belongs to Geoscience 331 at University of Wisconsin - Madison taught by Huifang Xu in Fall 2015. Since its upload, it has received 428 views. For similar materials see Gems: The Science Behind the Sparkle in Geology at University of Wisconsin - Madison.
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Date Created: 12/05/15
Geoscience 331 Final Exam Guide What is a gem? A gem is a naturally occurring material desired for its beauty, valuable in its rarity, and durable to give lasting enjoyment o There are many different types of gems, most of which are quite ordinary looking until they are enhanced Enhancement often starts with basic cutting and polishing, but subjecting a gem to heat or radiation can also produce desirable effects Some gems are synthesized or simulated for commercial purposes o Synthetic: a gem that is humanmade and has an identical crystalline structure to a natural gem. They are made of carbon, just like naturallyproduced gems o Simulant: a gem that is made to look like another, more valuable gem What makes a gem desirable? Gems are often enhanced to make them more desirable A common way to do this is to cut facets into it, increasing the brilliance, fire, and luster of a gem (and thus increasing it desirability) o Brilliance: amount of light reflected at the viewer; this is the gems’ “sparkle” o Fire: shows small bands of rainbow color in the gem, which are measured by dispersion Dispersion can be measured accurately, so gems that display lots of fire have a higher dispersion (and vice versa) o Luster: the surface appearance of a gem, and how light is reflected off it Some gems have a glassy luster (like glass), some have a waxy or metallic luster, etc. The degree of surface polish determines the type of luster on a gem (the smoother the surface, the more the luster will appear desirable) o Color: the more pure and transparent the color, the more valuable o Resisting to scratching (durability): evaluated gem hardness (the more resistant to scratches by dust, sand, etc., the more valuable) There are two measures of hardness scratch and indentation hardness (we are more concerned with scratch hardness) Anatomy of a cut gem Crown: top part of the gem Girdle: widest part of the gem; marks division between crown and pavilion Pavilion: lower portion of the gem Culet: bottommost facet of a gem (sometimes referred to as “cutlet”) Table: topmost facet of a gem Common types of cuts Cabochon: a round or ovalshaped stone that is flat on one side, often used with opaque gems such as opals and star sapphires Emerald: a boxyshaped cut often used to cut emeralds because of their tendency to be brittle (this cut helps to minimize chipping) Brilliant: a cut designed to maximize the amount of light reflected back to the viewer from the stone, often used with diamonds Value and the four C’s There are four key factors that affect the value of a gemstone Color: some colors are more desirable than others Clarity: cracks and inclusions (flaws) lower the value of a gem. The more clear a gem is, the more valuable it will be Cut: properly cut gems maximize fire, brilliance, durability, etc. Carat weight: a function of volume density (bigger is not always better) o 1 carat = 0.2g/5 carats = 1g Rarity can also be a factor in the value of a gem Supply of the controlling of supply can also change the value of a gem by changing demand or investment potential by consumers The value of a gem may also be lower if its clarity or color has been altered by treatment (heat, dyeing, etc.) Chemistry Review The Atom An element cannot be broken down into simpler thing An atom is the smallest particle that retains all the properties of an element o Atoms combine together to form compounds o Most minerals/gems are chemical compounds The nucleus is the center of an atom, and is where most of its mass lies o Protons and neutrons live within the nucleus, and each has a mass of 1 Protons have a charge of +1, while neutrons have no charge o A cloud of electrons surrounds the nucleus Electrons have very little mass, but have a charge of 1 o In most atoms, the number of protons and electrons are the same, so the net electrical charge of most atoms is zero Elements Elements are defined by the number of protons they have in their nucleus; each element has a different number (Hydrogen has one, Helium has two, etc.) Ions When an atom has a higher or lower number of electrons than its number of neutrons, the atom is called an ion o Strength of the atom’s charge is directly related to the number of protons and electrons If an ion has a positive charge, it is called a cation If it has a negative charge, it’s an anion o Anions are attracted to cations and vice versa (opposites attract) Oxidation States When an element has lost electrons, it is in a state of oxidation o Elements can have more than one oxidation state (they don’t need to lose a certain number of electrons) Isotopes An isotope is an atom with a different number of neutrons than what is normal A normal number of neutrons often means that the atom has the same number of neutrons as protons o Isotopes tend to be radioactive Chemical Bonds Ionic bonds are when two atoms combine with a net charge of zero o These are formed when the “extra” electron of one atom is attracted to another atom that needs an electron (ex: Sodium and Chloride; Na+1 + CL1) Covalent bonds are when atoms share electrons to gain stability; neither one of the atoms has a bigger “pull” on the electrons o The carbon atoms in diamonds are bonded by covalent bonds Geologic Principles The Earth Most of the earth is made of rock, nickel, and iron The outermost layer is called the crust o Very thin, the crust is the only layer we observe o The vast majority of gemstones are found within the crust o Oxygen and silicon are the two most common elements in the crust, so it’s not surprising that most gems are silicates (having silicon and oxygen) The mantle lies below the crust, and the crust sits atop the mantle in the form of plates o The mantle is not brittle like the crust, it will flow and bend under stress The core is mostly nickel and iron o The outermost layer of the core is molten, but the inside has enough pressure to keep the iron and nickel solid Minerals and rocks A mineral is a naturally occurring crystalline substance that is solid and inorganic with a specific chemical composition o Most gems are minerals, although minerals can have different gem names associated with them (same mineral; different colors) Rocks are combined groups of minerals, although some of them only contain one type of mineral o Some rocks are known to contain certain minerals (ex: igneous always contains quartz, feldspar, and mica) Specific areas of earth produce specific types of rock Igneous rocks and volcanic processes o Igneous rocks form from molten material and form most of the earth’s crust The source material for igneous rock comes from the mantle Partial melting causes it to rise to the surface, where it cools The speed at which the rock cools determine the size of crystals found in it (longer=bigger crystals) Volcanic rocks exposed at the surface cool quickly, while molten material in magma chambers will cool slowly These chambers where most large crystals are found are called plutons o After cooling, a small amount of material remains Water vapor, carbon dioxide, and sulfur dioxide are common gases found in magma, and are confined by the immense pressure of the toprocks The gases, called volatiles, remain part of the magma until it is almost completely crystallized These often play an important role in contact metamorphism o Pegmatites are exceptionally coarsegrained igneous rocks They contain crystals larger than a centimeter in diameter They form in the last stages of crystallization, then volatiles constitute a high percentage of the melt Ions easily move through this fluid, allowing the growth of large crystals Pegmatites often contain significant quantities of valuable materials Pegmatite crystals are often large, and the volatiles that are left will determine what types of gems form Sedimentary rocks o Sedimentary rocks occur when water washes away mineral material and it settles into layers The lower layers are eventually lithified into rocks through compaction and cementation The rocks are broken down by weathering, which also exposes and concentrates gems Gems are not often formed by weathering, but it is often by weathering that they are found Being more dense, gems are often concentrated by weathering in stream beds or along beaches These are called alluvial (or placer) deposits o Gems retrieved from these deposits are often rounded from weathering o Gems formed by sedimentary processes occur as a result of precipitation of dissolved minerals because the minerals from the water pile up in large deposits o When water collects nears magma chambers (at a high temperature and pressure), it contains minerals that might not normally be soluble in water o Hydrothermal deposits occur when a mineralrich solution encounters open spaces (such as cracks) and the gems crystallize (precipitate out of the solution) As a result, ‘veins’ of minerals fill the cracks Metamorphism Metamorphic rocks are changed by heat, pressure, and interaction with high temperature fluids o Can be considered as intermediary between igneous/sedimentary rocks o Some metamorphic rocks barely change; others look nothing like their parent rocks (the rocks or source they come from) The type of parent rock affects the type of metamorphic rock produced Contact metamorphism occurs when rocks touching molten magmas are altered o Involves high temperatures, but low pressure (ex: garnet) o When a body of magma intrudes into an area of limestone, rare and brightly colored minerals can form (these are skarn deposits) o When the magma also introduces new elements into the metamorphosed rock, it makes new minerals (this is metasomatism) Regional metamorphism is when deeply buried parent rocks are altered by increased pressure and temperature o This often results in flattened layers of minerals, which look banded or laminated (this texture is called foliation) o The type of rock that forms is a result of the temperature and pressure, while the types of minerals that form are result of the parent rock o Shales (clayrich sedimentary rocks) react strikingly to changes in pressure and temperature o As temperature and pressure increase, the amount of foliation and crystallization also increases Lower grade metamorphic shales produce slate (blackboard, roofing, etc.) Higher grade produces schists and neocrystallization (new crystals that form from parent rock) Highest grade produces gneiss Formed vs. Found: surface processes There’s often a difference between where a gem is formed and where it is found o Most are formed deep underground, where it is too hot to collect them o Most gems are found by mining Natural processes like faulting and volcanism can carry gems from deep within the earth Ex: rubies are often formed in igneous rock, but as they weather out of the rock, they are often found in alluvial deposits Hardness: Moh’s Scale Hardness is a gem’s resistance to scratching o The two measures of hardness are scratch hardness and indentation hardness Scratch hardness is used more often, and it has a scale from 1being the softest to 10 being the hardest Ex: Diamond cannot be scratched by quartz, so diamond is harder than quartz Cleavage and Fracture Minerals can break by fracture or by cleaving o Cleavage: tendency of a mineral to break to split along planes of weakness in a crystal More likely to split in minerals where atomic bonding is the weakest Ex: diamonds are hard, but are prone to splitting along specific cleavage planes Minerals without cleavage display concoidal fracture (like broken surfaces of window glass) Malleability is the property to bend, ductile is being able to be drawn into thin wire o Metals are malleable and ductile, while most gems are too brittle Specific Gravity Specific gravity: mass of the mineral compared to the mass of an equal volume of water o If mineral has the same mass as an equal volume of water, it has a specific gravity of 1 (less mass means a specific gravity <1; these compounds float) Specific gravity is related to density (more dense means higher specific gravity) Luster The surface appearance of a gem depends on the way light is reflected off the surface o Ex: glassy luster appears glassy, smooth, and shiny Luster descpriptions are as follows: o Metallic and submetallic (reflect the most light possible; not common in gems other than gold and silver) o Adamantine (luster like a diamond; highest refractive property, and is the highest possible luster for transparent gems) o Waxy (like a fingernail or candle; little light reflected) o Etc. Optical properties 1: Light transmission (proportional to the amount of light reflected off the surface, or absorbed/scattered inside the stone) o Optical terms include transparent (outline of an object is seen perfectly through a gem), translucent (outline of object is not seen, but light passes through the stone), opaque (no light gets through), etc. 2: Refractive index (reflection and refraction) o Rays of light can be reflected off a gem or can pass through the surface of a gem If the light bounces off the surface, it is reflected (the angle of incidence equals the angle of reflection) If light passes from one material to another, it is refracted The amount light is bent depends on the density difference between the gem and air The measure of refraction is the refractive index (RI) o When light enters a substance with a higher RI, the light bends towards a reference line called the Normal The Normal is a line perpendicular to the surface of the gem (moving to materials with a low RI, light will bend away from the Normal) The “bend” depends on number, size, and arrangement of atoms3. Snell’s Law (the relationship between the angle of incidence and angle of refraction) o RI = (sin i)/(sin r) Ex: Ray A enters a gem at 60 degrees; B at 30 degrees In A, i = 60 and r = 21 (B; i = 30 and r = 11.94) A: RI = (sin 60)/(sin 21) = 0.8660/0.3583 = 2.417 o **What is B? (Test your knowledge)** 4. Critical angle (the angle at which total internal reflection is achieved, meaning light is refracted so much it is refracted back into the stone and not out of the other side) o The application of this property is what makes gems sparkle o The critical angle determines how facets are be placed in order to control the path of light in a gemstone In order to achieve brilliance and sparkle, light must not to escape from the pavillion (base of the stone) The light should reflect inside the stone, and leave from the top facets 5. Fire and Dispersion o Fire is the rainbowlike flashes seen in cut stones, and is due to the difference in refraction between wavelengths (colors) of light Violet light is bent more than red light, and dispersion (which causes fire) is the amount of refraction of violet light minus the refraction of red light Greater difference = greater dispersion o Dispersion can be accurately measured, and is a helpful diagnostic property Dispersion is valued in gemstones, but is ruins cameras and telescope lenses (less dispersion allows a lens to focus) Other Optical Phenomena 1. Adularescence: a milky sheen created by tiny particles or irregularities in the crystal structure of the gem (ex: moonstone) 2. Interference and Diffraction Colors o Occurs when individual color wavelengths cancel each other out, or amplify one another (why compact discs have a rainbow effect) o Play of color occurs when certain wavelengths are reflected out of the stone, producing flashes of color Iridescent color, or labradorescence, or schiller arises most often in the feldspar variety labradorite and fire opal 3. Color change o Not all light is pure light (candlelight has more red wavelengths, fluorescent lights have more blue, etc.) o Some gemstones will selectively reflect the colors of the light illuminating it Ex: alexandrite (red in candlelight, blue in fluorescent light) 4. Chatoyancy and Asterism o Chatoyancy occurs in gemstones with long linear inclusions If cut in a cabochon shape, the surface will reflect light at right angles to these long fibrous inclusions (ex: tiger’s eye) o Asterism is the same, but the inclusions are hexagonal (produces a sixrayed star) 5. Aventurescence o Some stones produce a glittery effect as light bounces off small reflective inclusions within the stone (aventurescence; ex: bloodshot iolite) Sometimes the glitter is very finely distributed which gives an overall metallic sheen rather than individual sparkles (ex: sunstone) 6. Pleochroism o Pleochroism: when a crystal changes color based on the direction in which light passes through it (ex: tanzanite) 7. Fluorescence and phosphorescence o Fluorescence occurs when an ion absorbs ultraviolet light (UV) energy and then releases this energy as visible light: the visible light appears as the “highlighter colors” If fluorescence occurs after removal of exciting radiation, it is phosphorescence o In gemstones, these ions are impurities (often transition metals such as manganese) whose electrons absorb the energy from UV light The electrons "jump" up to a higher energy level and then "fall" back to their original (ground) state As they go back to a lower energy level, they emit energy o Some energy is escapes as heat, the rest as visible light These impurity ions are called activators In some gemstones, structural defects in the crystal structure behave as activators o Different ions vary in their ability to absorb certain wavelengths of UV light Some stones only fluoresce under shortwave UV (SWUV = 253.7 nanometers), some only under longwave UV (LWUV = 365.4 nanometers), while others fluoresce under both wavelengths o The color a gem fluoresces is not always the same as its color under normal light Some gems have a stronger reaction under one type of UV light, and some are equally intense o High energy (short wavelength) UV light excites electrons in the activator and raises them to a higher energy level When the electrons drop back to the ground state, they emit energy at lower wavelengths (mostly as visible light) o Phosphorescent materials have a time lag between the excitement of the electrons and their drop back to normal energy levels (they "glow in the dark") o The amount and nature of the activators have little or no affect on a gem's fluorescence Of two gems that come from the same area (even the same vein or pocket), one may fluoresce strongly, while the other does not Some fluorite will fluoresce, but not all o Few minerals have a constant (and therefore diagnostic) fluorescence Thus, fluorescence is not a diagnostic identification, but it only part of more accurate testing methods How do we measure refractive index? Gemologists measure RI with a refractometer, which measures a gem’s critical angle o The gem (with a flat faceted surface) is placed on the testing surface A ray of light is shown at the gem, and the point at which it is reflected off the surface is carefully measured There are several other handheld tests that use the concept of the critical angle to identify real versus imitation gemstones What is Color? Visible light is only a small part of the electromagnetic spectrum o Radio waves, light waves, and cosmic rays make up a spectrum of energy o Visible light occupies approximately the middle of the spectrum from 400 to 700 nanometers (billionths of a meter nm) Each color has its own range of wavelengths o Red light occupies 630700 nm, while Violet light has wavelengths of 400430 nm, etc. o Photons with short wavelength (e.g., UV light) have high energy (e.g., red light) Our perception of color occurs when specific of wavelengths of light strike certain cells on the back of our eyes o If light of 700nm wavelength were to hit our eyes, we would perceive it as red light, just as we would perceive 400nm wavelengths to be violet Light from several wavelengths together form other colors o All wavelengths together form white light, while no light is black However, that this is not quite the same principle as additive mixing in paint pigments If an object absorbs wavelengths of light and reflects only specific ones to our eyes, the object will appear to be that color o Thus if a gem reflects light of 700630 nm, it will appear red (and so on) o Herein lies the most important concept behind color in gemstones Gems have different colors because they reflect specific wavelengths of light Why Gems have Color Chromophores o The configuration of ions in crystal structure has a large influence on the selective absorption of light wavelengths (and hence color) Certain ions are more likely to absorb a light hitting the crystal structure o Transition metals have electron configurations conducive to this phenomenon These elements have a very strong influence over the color of a gemstone The ions of these elements that produce color are called chromophores These coloraffecting elements include: titanium, vanadium, chromium, manganese, iron, cobalt, nickel, and copper Remember these! While the chromophore ion itself absorbs light, neighboring ions influence the way light is o The chromophore ion absorbs light, but other ions influence the way light is absorbed A chromophore in one mineral might produce another color in another mineral o Color also depends upon the oxidation state of the chromophore Ex: In peridot (olivine), the presence of ferrous iron (Fe 2+) produces a green color, while ferric iron (Fe 3+) produces a yellow color in chrysoberyl Idiochromatic minerals o In some minerals, color arises from a major constituent element Since the mineral contains a great deal of this element, the color never changes These are idiochromatic minerals o In these minerals, color is an important identifying trait For example, malachite is always green and azurite is always blue (both contain the Cu2+ ions) Allochromatic minerals o In many gems, the elements composing these minerals have no characteristic color Often the pure form will be white or clear (like pure quartz) o Colored varieties of these gems can be more common than the pure forms Ex: White nephrite jade is less common than green nephrite Iron will substitute for magnesium in the jade crystal structure to produce the green color o As the amount of iron increases, the color will change from light green to dark green to black o Minerals that show variation in color are called allochromatic Substitutions occur both with major element substitutions and also minute amounts of impurities that enter into the crystal structure Ex: Corundum is colorless, unless a small amount of chromium (Cr3+) ions are present (the corundum then turns red and is called a ruby) o If iron and titanium enter into the structure, corundum becomes blue instead producing deep blue sapphires o As stated earlier, the surrounding crystal structure also affects the way chromophores absorb light Ex: chromium impurities will produce a red color in rubies, but will produce a deep green in beryl (to produce emerald) Color centers o Another way for a gem to absorb specific light wavelengths is a color center o Color centers are imperfections in the lattice of the crystal o Imperfections can be due to a number of factors: Excess of one element Lack of another element Substitutional impurities different ions that "push" their way into the lattice Mechanical deformation of the crystal lattice Can be caused by heat, pressure, or even radiation Whatever the cause of the color center, the spot where an anion should be (but is missing due to the defect) can act as an electron trap o This electron can then act as an absorber of light Inclusions: color invaders o Color can arise from bits of other minerals, inclusions, that are incorporated within the gem's crystal (many quartz varieties get their color this way) Chalcedony varieties jasper, fuschite, and aventurine all get their color from inclusions of other minerals Variation in Color Minerals don't always have constant color throughout, either o Many times color is irregularly distributed within the gem, creating a somewhat patchy appearance Sometimes the growth pattern of a crystal will create distinct bands of color o As the gem crystal grows, specific bands or areas of color will form Some color patterns can be diagnostic, especially for nontransparent minerals o Some color banding is indicative of a synthetic gemstone, such as curved color bands in a sapphire a natural stone would show straight or hexagonal bands Tourmaline o Tourmaline exhibits some fascinating color variation Some varieties of tourmaline vary along the length of the crystal Varying from green at the base to colorless, to red at the top There are also color variations that occur from the inside out red in the middle, green on the outside This variation has to do with the growth of the crystal and the presence of certain transition metals during phases in the crystal's formation Quartz o Ametrine is a bicolored quartz with amethyst and citrine on either end Amethyst will show distinctive bands of color that follow the crystal growth lines This is diagnostic an evenly colored amethyst is probably either synthetic, or not amethyst at all Malachite o Malachite shows variation in the shade of green that it exhibits This is due to the different crystal sizes Smaller crystals reflect light differently and look lighter than larger crystal bands Color Enhancement Efforts to improve the color of a gem address many of these issues o There are many ways to alter the color of a gem o Before, the common method was to dye or apply colored oil to the surface of a stone Other more complicated procedures involve radiation and/or heat to alter the color centers within a stone Sometimes high pressure can change the crystal lattice, and alter the color "Oiling" is a common method of gem enhancement o A thin layer of oil inside the cracks of a gemstone even out the reflected light, and make the gem look more brilliant and also improve the clarity The light reflected in all directions on the left makes the surface appear dull, but the light reflecting off in the same direction makes it appear glossy There are nearly as many ways to enhance color as there are gems o The goal of color enhancement is to emphasize the traits that produce desirable color in gems, while reducing the aspects that might reduce the quality of color in gemstones Ex: sapphires are routinely heat treated the high temperatures dissolve rutile inclusions within the gem This serves two purposes it makes more titanium ions available to act as chromophores and improves the clarity by dispersing the inclusion Internal arrangement All minerals and crystalline and have a unique crystal structure o The crystal structure of a mineral is often the same some minerals can have multiple crystal structures; these are polymorphs o A pseudomorph is a mineral that forms in the shape of another crystal The shape of a crystal shows the internal composition of a mineral o Ex: cubeshaped crystals have cubeshaped molecular structure, hexagonal crystal have hexagonal minerals, etc. Symmetry All minerals have a distinctive crystal shape (some have more than one) o Since crystal shape is often dependant upon chemical makeup, crystals can be classified and described based on the symmetry of their crystal shape All crystals have symmetry some more than others There are six groups of crystals based on symmetry (called systems) o The groups are listed below in decreasing order of symmetry: Cubic Tetragonal Orthorhombic Hexagonal (includes Trigonal) Monoclinic Triclinic The Unit Cell: It All Begins Here The unit cell is the building block that all crystals are made of A unit cell can be an atom, a molecule, or a group of molecules It is the smallest unit that keeps the symmetry of the crystal It’s repeated over and over again to make a crystal Unit cells are repeated in various patterns to make crystal o Ex: a cubic shape unit cell can be stacked together to form other shapes (for example, cube blocks stacked together to create pyramids in Egypt) In this case, the unit cell is the pyramid, not the cube Anatomy of a Crystal Axes (ex: north and south pole axis of Earth): o In a cube, the axes are the same length, in other crystal shapes, one axis is longer than the others The longest axis is the caxis Faces: o While talking about crystals, we’ve been referring to wellformed single crystals o Euhedral crystals are rare minerals grow in the space available Sometimes crystals have grown together so that some lattice directions are parallel, while the other lattice directions are in reverse (twinning) Faces and facets cut gems versus crystals o Crystal faces are flat surfaces that form naturally as a result of the structure and growth of the mineral o Facets are flat surfaces created by gem cutters to increase the value of a gemstone Crystal faces don't always match up with gem facets, partially because gems are cut not only to increase its value, but also to protect the gem from breaking or chipping o Many times, the gem minerals that are fashioned into cut stones are anhedral crystals that do not have much value for the inherent crystal shape Sometimes a perfectly formed crystal is worth more untouched than as a faceted gem There are many other factors influencing how and why a gem is cut (will be covered later) Quartz The most abundant elements in the earth’s crust are oxygen (O) and silicate (Si) Silicates are the largest and most varied groups of minerals, and they are all based on the siliconoxygen tetrahedron (SiO4), which is the building block for all silicates Quartz is the most basic (SiO2) o It is colorless when pure, but impurities create all the colors we associate with quartz Quartz basic data: o Chemical Formula: SiO2 o Mohs' hardness : 7 o Crystal System: Hexagonal or trigonal o Color: Many (see varieties) o Fracture: Conchoidal o Specific Gravity: 2.65 o Refractive Index Low: 1.54 o Luster: Vitreous (Glassy) o Interesting Property: Piezoelectric (forms an electric charge when pressure is applied to it; does funny things when given a charge) Quartz will flex when given a charge (alternating currents will cause it to move back and forth) Quartz is found in igneous, sedimentary, and metamorphic rocks o It is hard, lacks cleavage, and is chemically stable (thus it does not weather) Flint (quartz) was the first mineral fashioned into tools and weapons Varieties: o Quartz is divided into two broad categories Coarsely crystalline: Referred to as “quartz” Cryptocrystalline: Referred to as “chalcedony” or “chert” o These categories differ in formation, grain size, impurities, etc. Coarsely Crystalline Varieties Ex: rock crystal, amethyst, citrine, smoky quartz, etc. o Color depends on impurities Rock crystal: colorless/not often used as a gem (clear and can be used in microscopes) Amethyst: iron gives amethyst its characteristic color (in cut stones, alternating bands of colors may be seen) o Heat treatment is often used to remove a smoky appearance Citrine: transparent orange and yellow quartz (also demonstrates zonation in color; can be created by heat treating amethyst) Smoky Quartz: grey/black quartz (gets its color from aluminum/oxygen charge transfer) Rose Quartz: pink comes from small amounts of titanium Milky Quartz: appearance due to inclusion (often of fluid) Rutilated Quartz: normally clear quartz that contains fine rutile crystals (TiO2) Cryptocrystalline Quartz Fibrous varieties: o Carnelian (orange/brownred from hematite), chrysoprase (yellow from nickel), agate (layered bands of varying colors) Granular varieties: o Jasper (opaque red, green, blue, or black from ironoxide), bloodstone (jasper with green body color and red inclusions) Opal Opal is an amorphous silicate (no crystalline structure yet displays internal order) o Formed from balls of amorphous silica (size/spacing of balls affect wavelengths of light) Opal Varieties (three groups) o Precious Opal (two subgroups) Display opalescence (spectral color, iridescence changes with angle) White Opal: An opaque stone in which colors appear as flashes or speckles Black Opal: Contains fire with a dark body color (less common; more costly) o Fire Opal Transparent or translucent with an orange or red body color (named for their color, but are not opalescent) o Common Opal Opaque (many names used to describe varieties) Opal is formed by circulating groundwater or lowtemperature hydrothermal solutions o It lines/fills cavities in rock near the surface o India and Eastern Europe were historically the sites to find opal, but now most of it comes from Australia Opal Enhancements o Doublets and Triplets: a thin slice of opal cemented to a dark substrate (less expensive than precious opal) o Opal treatments: surface oiling, heating etc. o There is both synthetic opal and stimulants of opal (glass “slocum stone”) Olivine (Peridot) Basic Data: o Chemical Formula: (Mg, Fe)2SiO4 o Mohs' hardness: 6.57 o Crystal System: Orthorhombic o Color: Yellowgreen to green o Fracture/cleavage: Conchoidal/poorvery poor o Specific Gravity: 3.274.37 o Refractive Index: 1.69 o Luster: glassy o Interesting Property: Major component of the mantle Olivine is a name for a range of chemical compositions and colors, however it is often a pale green (higher iron content = darker color) o Peridot is the gem form of olivine Origin and occurrence o Found mostly in igneous rocks, olivine is common in basalts and peridotites (peridotrich), which contain very little (if any) quartz “Dunite” is a rock made almost entirely of olivine Peridot is common in Hawaii o Olivine has a high melting point and is one of the first minerals to crystallize from molten rock Mgrich olivine crystallizes first, the Ferich ones o Olivine can exhibit color zonation (tend to have Mgrich cores which are pale colored, and Ferich rims which are more brown) Pyroxenes Basic Data: o Chemical Formula: LiAlSi2O6 o Mohs' hardness: 6.57 o Crystal System: Monoclinic o Color variable: white, gray, pink, yellow, green o Fracture/cleavage: Conchoidal/good o Specific Gravity: 3.153.2 o Refractive Index: 1.67 o Luster: Vitreous o Interesting Property: Crystals up to 40 feet in length found in the black hills of SD Pyroxenes are silicate minerals with a wide variety of compositions o The simplest pyroxenes are MgSiO3 The silica tetrahedrons form long chains, which are made of only one row of silica each (unlike amphibole) Pyroxenes are not resistant to Earth’s physical and chemical processes o They react to rain, weak acids, and even physical abrasion o Pyroxenes combine with carbonic acid to form clay Pyroxenes usually crystallize from an igneous melt, but can form in hightemperature metamorphic rocks o Pyroxene and olivine both have high melting points, and are often found together Spodumene or Kunzite o Lithium (Li) bearing pyroxene (Li is the lightest metal) o Color is light pink/purple o Some crystals are large; usually found in pegmatites Irradiation causes them to go yellow/green (unstable color) Diopside (Calcic magnesian monoclinc pyroxene) o Common mineral (less commonly used in jewelry) o Sometimes displays asterism Augite (calcic clinopyroxene) o Another common pyroxene, whose composition is similar to diposide Hiddenite o Chromium (Cr) rich pyroxene Strongly pleochroic o Good cleavages makes gems difficult to cut Amphiboles Basic Data: o Chemical Formula: Ca2(Mg, Fe)g5Si8O22(OH)2 o Mohs' hardness: 56 o Crystal System: Monoclinic o Color: white to light green o Fracture/cleavage: Conchoidal/good o Specific Gravity: 3.03.2 o Refractive Index: 1.61 o Luster: vitreous o Interesting Property: amphibole group includes minerals that form asbestos Amphiboles have similar chemical structure to pyroxenes, but are more complex o Unlike pyroxenes, the silica tetrahedrons are arranged in doublewide chains This affects the physical properties of amphiboles o Not many important gem forms of amphiboles exist, but several varieties of asbestos are amphiboles o When quartz replaces crocidolite (a form of asbestos) fibers, the fibrous shape results in a highly sought after effect in quartz Ex: Tiger's eye quartz exhibits chatoyancy (silky sheen), and is a result of the quartz retaining the fibrous structure of crocidolite Jade “Jade” refers to two separate minerals: jadeite (a pyroxene) and nephrite (an amphibole) o Both often occur as a semitranslucent green mass of small, interlocking crystals This forms a very tough aggregate The tightly interlocking crystals makes is resistant to fracture (it was often used as ancient tools/weapons) Jadeite o Sodium (NA)Aluminum (Al) silicate (often used for carvings) o Moh’s hardness: 6.57 o Specific Gravity: 3.4 o Crystal System: Monoclinic Formed of small, interlocking crystals o Cleavage: at 90 degrees; not seen in massive form o Color (due to Cr + 3): deep green Fe produces a paler color o Formed by highpressure metamorphism of Narich rocks Found in North Burma, Guatemala, Japan, etc. o “Imperial” or “gem” jade is highly valued vivid green variety Nephrite o Amphibole, not pyroxene (double chain silicate) Unlike pyroxenes, amphiboles are hydrous materials (contain water) o Used for carvings in places like China and Central America o Composition: calcium (Ca) and Magnesium (Mg) silicate o Green color is due to Fe Greatly resembles jadeite in color and structure o Moh’s hardness: 66.5 Harder than steel (used in Neolithic tools) o Specific gravity: 2.95 o “New Jade” = serpentine (MH = 5.56; SG = 2.5) o “Styrian Jade” = chlorite Feldspars Basic data: o Chemical Formula: Potassium, Sodium Aluminosilicates o Mohs' hardness: 6 o Crystal System: Monoclinic (Orthoclase), Triclinic (Plagioclase) o Color: (see varieties) o Fracture: Irregular, good cleavage in two directions o Specific Gravity: 2.57 (orthoclase) o Refractive Index: 1.52 (orthoclase) o Luster: vitreous o Interesting Property: Most abundant mineral in the Earth's crust Feldspars area framework silicates because of the tetrahedra (silicon surrounded by four oxygen ions) being linked together in three dimensions to form a complex framework Plagioclase feldspars o Rich in Na and Ca (sodium, calcium, and aluminum silicates) Ex: Labradorite, anorthite Akali feldspars o Sodium, potassium, aluminum silicates Ex: Moonstone, amazonite, etc. Where are feldspars formed? o Feldspars make up 5060% of the volume of crustal rocks, and are found in many different rock types Common in igneous rocks, and sometimes form huge crystals (indicating that they have cooled slowly far beneath the surface of the earth) o Feldspars are very important and interesting minerals but less important as a source of gemstones o Relatively susceptible to weathering and react with mild acids to produce clay Other interesting characteristics o Plagioclase feldspars Commonly twinned (twin planes produce parallel striations on mineral's surface) Gem varieties: Labradorite (dark grey color; iridescent) Play of color is referred to as 'labradorescence' Some wavelengths are amplified, and some are cancelled Net result is that different spacings/orientations produce rainbowlike effects Iridescence is mostly blue, but often with rainbowlike appearance o Akali feldspars Gem quality orthoclase: yellow stones colored by Fe impurities Microcline Amazonite is a light greenblue form of microcline Garnet Basic data: o Chemical Formula: varies (see below) o Mohs' hardness: 6.57.5 o Crystal System: Cubic o Color: Any except blue (see below) o Fracture: Conchoidal o Specific Gravity: 3.54.3 o Refractive Index: 1.711.89 o Luster: vitreous to resinous o Interesting Property: from Latin Granatum ("seed like" after pomegranate seeds) Garnets are silicate minerals with diverse compositions o All garnets have almost identical atomic structures The term garnet applies to fifteen distinct minerals, five of which are common gemstones o Garnets can appear nearly every color except blue o Garnets do not have cleavage The generalized chemical formula of garnets is X3Y2 (SiO4)3 where: o X indicates a divalent cation, such as iron(Fe2+), magnesium (Mg2+), calcium (Ca2+) or manganese (Mn2+) o Y is a trivalent cation, such as aluminum (Al3+), iron (Fe3+), or chromium (Cr3+) o The SiO4 indicates silica tetrahedrons Garnets belong to the Cubic crystal system o Garnets often grow in a distinctive welldeveloped crystal form known as a dodecahedron (12 sided, usually have triangular or rhomboid shaped faces) o Because the atoms are tightly packed, garnets are relatively hard and dense Physical Properties vary with composition they are all different o Garnets break down into two main groups: Alrich garnets and Carich ones Most have compositions that involve complex mixtures of cations Because of their variable composition, garnets may have almost any color o Al rich garnets: Pyralspite/Ca rich garnets: Ugrandite The ugrandites are rarer than the pyralspites Grossular: In its purest form, this calcium aluminum silicate is colorless, but displays a wide range of color depending on the impurities (mainly Fe) present o Two main color varieties are Tsavorite and Hessonite Tsavorite a green grossular (second only to Demantoid in value) Hessonite is a variety of Grossular: greenyellowbrown Andradite o This calcium iron silicate is often yellow, black, or brown and is not commonly used for gems (except the green variety Demantoid) Demantoid (Crandradite): rare, vivid green garnet color (most valuable) Colored by Cr / V (thus, Ca Al,V) It has more fire than diamond! (but it's much softer) Sometimes chatoyant: may contain "horsetail" inclusions (mostly from Urals) Spessartine: Orange colored garnets (Fe, Mn, Al) Pyrope: Brownred ("cape ruby") Almandine: Violetred Uvarovite: Emeraldgreen garnet Rhodolite: Purple red Geological locations o Garnets can be found in a variety of rocks (igneous, metamorphic, and sedimentary) o Ironrich almandine, the most common garnet, is widespread in metamorphic rocks such as schists and gneisses and may occur in granitic igneous rocks o The magnesium garnets, are formed in high pressure environments and are found in magnesium rich metamorphic rocks formed at great depth They may be an important mineral in the mantle of the Earth o Spessartine is found in manganeserich gneisses and in coarse grained, igneous rocks (pegmatites) o Grossular, containing calcium and aluminum, is found in clay rich limestones that have been metamorphosed to marble and in contact metamorphic deposits, (skarns), formed when an igneous rock intrudes and reacts with limestone o The calciumiron garnet andradite and the rare calciumchromium garnet uvarovite are also usually found in skarns They are also formed by regional metamorphism, especially of sedimentary rocks such as limestone Other interesting facts o Garnets have been a prized gemstone for over 5000 years The Romans used them for carvings They were a particularly popular gemstone in the 19th century o Limited availability of natural gem quality garnets keeps them from being commonly used today Nongem quality almandine garnets are sometimes used as abrasives especially in garnet sandpaper o Demantoid is so named from the Old German word Demant, which means "Diamond” Demantoid has a higher dispersion than diamond and adamantine luster Gems larger than a few carats are quite rare and it is still a valuable gem today The other color varieties of andradite garnets are brown, yellow and black, but these do not often become gemstones Topaz Basic data: o Chemical Formula: Al2SiO4(F,OH)2 o Mohs' hardness: 8 o Crystal System: Orthorhombic o Color: varies see below o Fracture: Conchoidal, perfect basal cleavage o Specific Gravit: 3.43.6 o Refractive Index: 1.61 o Luster: vitreous o Interesting Property: Largest faceted gem weighs over 10 pounds Color: reddishyellow ("imperial"), orangebrown ("sherry"), clear, pinkish, natural blue (light) and treated blue (darker) Geologic conditions of formation o Topaz crystallizes from fluorinebearing vapor in last stages of solidification of igneous rocks Occurs within cavities in lavas and granitic rocks, primarily in pegmatites Here, large cavities and slow growth can produce huge crystal o Secondary concentrations of topaz occur in stream beds and other alluvial deposits They don't form in the stream, but are concentrated in stream gravels by the moving water Topaz is the most common irradiated gem on the market today o This is primarily due to consumer demand for the deep blue shades o Particles emitted by radioactive decay, or electromagnetic rays (ionizing radiation), have enough energy to produce color centers within the gem's crystal structure It is likely that all dark blue topaz has been irradiated (and often subsequently heat treated) Most pink topaz has been "pinked" (heat treated to remove a yellowbrown color center) Radiation and Gemstones The use of radiation has only begun relatively o Artificial radiation involves bombarding the gem material with a great deal of high energy radiation the type found in linear accelerators, nuclear reactors, and xray machines The growing public concern over excess radiation (as it can damage cell tissue, DNA, etc.) prompted the government to step in with regards to irradiated gems Radiation o Radiation is just energy emitted either as particles or as electromagnetic radiation (photons) o When we think of "bad" radiation, we are usually referring to ionizing radiation This is radiation th
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