Lecture 3 Notes (Geoscience 331)
Lecture 3 Notes (Geoscience 331) Geoscience 331
Popular in Gems: The Science Behind the Sparkle
Popular in Geology
This 5 page Class Notes was uploaded by Hannah James on Sunday September 20, 2015. The Class Notes belongs to Geoscience 331 at University of Wisconsin - Madison taught by Huifang Xu in Fall 2015. Since its upload, it has received 108 views. For similar materials see Gems: The Science Behind the Sparkle in Geology at University of Wisconsin - Madison.
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Date Created: 09/20/15
thsical Properties of Gemstones Lecture 3 Hardness Moh s Scale 0 Hardness is a gem s resistance to scratching O The two measures of hardness are scratch hardness and indentation hardness I Scratch hardness is used more often and it has a scale from lbeing the softest to 10 being the hardest I EX Diamond cannot be scratched by quartz so diamond is harder than quartz Cleavage and Fracture 0 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 I More likely to split in minerals where atomic bonding is the weakest 0 EX diamonds are hard but are prone to splitting along specific Cleavage planes I Minerals without Cleavage display concoidal fracture like broken surfaces of window glass 0 Malleability is the property to bend ductile is being able to be drawn into thin wire 0 Metals are malleable and ductile while most gems are too brittle Specific Gravity 0 Specific gravity mass of the mineral compared to the mass of an equal volume of water 0 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 lt1 these compounds oat 0 Specific gravity is related to density more dense means higher specific gravity Luster 0 The surface appearance of a gem depends on the way light is re ected off the surface 0 EX glassy luster appears glassy smooth and shiny 0 Luster descpriptions are as follows 0 Metallic and submetallic re ect the most light possible not common in gems other than gold and silver 0 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 re ected O Etc 5 Optical properties 0 1 Light transmission proportional to the amount of light re ected off the surface or absorbedscattered inside the stone 0 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 re ection and refraction We Re ec on Re ac nn An example of re ection vs refraction 0 Rays of light can be re ected off a gem or can pass through the surface of a gem I If the light bounces off the surface it is re ected the angle of incidence equals the angle of re ection I If light passes from one material to another it is refracted 0 The amount light is bent depends on the density difference between the gem and air I The measure of refraction is the refractive index RI 0 When light enters a substance with a higher R1 the light bends towards a reference line called the Normal I 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 0 The bend depends on number size and arrangement of atoms lllll39l39l 39 391 It a r u II r Inal 539 g In iT 39l a I rang 395 I lean I l i1iJL IE U3H3 derae 1iiui EDLJHIIEII UJ I 2 Hedi um h undgif E rill An example of light moving to high RI and to low RI 3 Snell s Law the relationship between the angle of incidence and angle of refraction 0 RI Sin iSin r I EX Ray A enters a gem at 60 degrees B at 30 degrees 0 InAi 6O andr21 Bi30 andr 1194 0 A R1 sin 60sin 21 O8660O3583 2417 O gt What is B Test your knowledge 4 Critical angle the angle at which total internal re ection is achieved meaning light is refracted so much it is refracted back into the stone and not out of the other side 0 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 I In order to achieve brilliance and sparkle light must not to escape from the pavillion base of the stone I The light should re ect inside the stone and leave from the top facets HID rmal This image demonstrates the di ferece between an average refractive angle and the critical angle 0 5 Fire and Dispersion 0 Fire is the rainbow like ashes seen in cut stones and is due to the difference in refraction between wavelengths colors of light I 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 0 Greater difference greater dispersion O Dispersion can be accurately measured and is a helpful diagnostic property I Dispersion is valued in gemstones but is ruins cameras and telescope lenses less dispersion allows a lens to focus Other Optical Phenomena I l Adularescence a milky sheen created by tiny particles or irregularities in the crystal structure of the gem ex moonstone 2 Interference and Diffraction Colors 0 Occurs when individual color wavelengths cancel each other out or amplify one another why compact discs have a rainbow effect 0 Play of color occurs when certain wavelengths are re ected out of the stone producing ashes of color I Iridescent color or labradorescence or schiller arises most often in the feldspar variety labradorite and fire opal 3 Color change 0 Not all light is pure light candlelight has more red wavelengths uorescent lights have more blue etc 0 Some gemstones will selectively re ect the colors of the light illuminating it I EX alexandrite red in candlelight blue in uorescent light 4 Chatoyancy and Asterism O Chatoyancy occurs in gemstones with long linear inclusions I If cut in a cabochon shape the surface will re ect light at right angles to these long fibrous inclusions ex tiger s eye 0 Asterism is the same but the inclusions are hexagonal produces a six rayed star Example of Tiger s EyeStar Sapphire chatoyancyinclusion and triple chatoyancy three inclusions 0 5 Aventurescence 0 Some stones produce a glittery effect as light bounces off small re ective inclusions within the stone aventurescence ex bloodshot iolite I Sometimes the glitter is very finely distributed which gives an overall metallic sheen rather than individual sparkles ex sunstone 0 6 Pleochroism O Pleochroism when a crystal changes color based on the direction in which light passes through it ex tanzanite 0 7 Fluorescence and phosphorescence 0 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 I If uorescence 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 I The electrons quotjumpquot up to a higher energy level and then quotfallquot back to their original ground state 0 As they go back to a lower energy level they emit energy 0 Some energy is escapes as heat the rest as visible light I These impurity ions are called activators 0 In some gemstones structural defects in the crystal structure behave as activators 0 Different ions vary in their ability to absorb certain wavelengths of UV light I Some stones only uoresce under short wave UV SWUV 2537 nanometers some only under longwave UV LWUV 3654 nanometers while others uoresce under both wavelengths O The color a gem uoresces is not always the same as its color under normal light I Some gems have a stronger reaction under one type of UV light and some are equally intense 0 High energy short wavelength UV light excites electrons in the activator and raises them to a higher energy level I 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 quotglow in the dar quot O The amount and nature of the activators have little or no affect on a gem39s uorescence I Of two gems that come from the same area even the same vein or pocket one may uoresce strongly while the other does not 0 Some uorite will uoresce but not all 0 Few minerals have a constant and therefore diagnostic uorescence I Thus uorescence is not a diagnostic identification but it only part of more accurate testing methods 7 What does all this have to do with gem cutting 0 Gem cutters need to understand how light behaves once it passes into a gemstone 0 They use the critical angle to determine how facets should be placed in order to control the path of light in a gemstone O In order to achieve brilliance and sparkle we want light to escape the top facets not the pavilion i lrm Hinzul alum I Examples of facets cut into gems to re ect light out of the top facets 8 How do we measure refractive index 0 Gemologists measure RI with a refractometer which measures a gem s critical angle 0 The gem with a at faceted surface is placed on the testing surface I A ray of light is shown at the gem and the point at which it is re ected off the surface is carefully measured 0 There are several other hand held tests that use the concept of the critical angle to identify real versus imitation gemstones