Lecture 4 Notes (Geoscience 331)
Lecture 4 Notes (Geoscience 331) Geoscience 331
Popular in Gems: The Science Behind the Sparkle
Popular in Geology
This 4 page Class Notes was uploaded by Hannah James on Thursday September 24, 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 39 views. For similar materials see Gems: The Science Behind the Sparkle in Geology at University of Wisconsin - Madison.
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Date Created: 09/24/15
II Color in Gemstones Lecture 4 What is Color Visible light is only a small part of the electromagnetic spectrum 0 Radio waves light waves and cosmic rays make up a spectrum of energy 0 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 0 Red light occupies 630700 nm while Violet light has wavelengths of 400430 nm etc 0 Photons with short wavelength eg UV light have high energy eg red light Our perception of color occurs when specific of wavelengths of light strike certain cells on the back of our eyes 0 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 0 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 re ects only specific ones to our eyes the object will appear to be that color 0 Thus if a gem re ects 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 re ect specific wavelengths of light Why Gems have Color Chromophores O The configuration of ions in crystal structure has a large in uence on the selective absorption of light wavelengths and hence color I Certain ions are more likely to absorb a light hitting the crystal structure 0 Transition metals have electron configurations conducive to this phenomenon I These elements have a very strong in uence over the color of a gemstone 0 The ions of these elements that produce color are called chromophores I These coloraffecting elements include titanium vanadium chromium manganese iron cobalt nickel and copper I Remember these While the chromophore ion itself absorbs light neighboring ions in uence the way light is 0 The chromophore ion absorbs light but other ions in uence the way light is absorbed I A chromophore in one mineral might produce another color in another mineral 0 Color also depends upon the oxidation state of the chromophore I 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 0 Idiochromatic minerals 0 In some minerals color arises from a major constituent element I Since the mineral contains a great deal of this element the color never changes 0 These are idiochromatic minerals O In these minerals color is an important identifying trait I For example malachite is always green and azurite is always blue both contain the Cu2 ions 0 Allochromatic minerals 0 In many gems the elements composing these minerals have no characteristic color I 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 I EX White nephrite jade is less common than green nephrite 0 Iron will substitute for magnesium in the jade crystal structure to produce the green color 0 As the amount of iron increases the color will change from light green to dark green to black 0 Minerals that show variation in color are called allochromatic I Substitutions occur both with major element substitutions and also minute amounts of impurities that enter into the crystal structure 0 Ex Corundum is colorless unless a small amount of chromium Cr3 ions are present the corundum then turns red and is called a ruby 0 If iron and titanium enter into the structure corundum becomes blue instead producing deep blue sapphires 0 As stated earlier the surrounding crystal structure also affects the way chromophores absorb light I EX chromium impurities will produce a red color in rubies but will produce a deep green in beryl to produce emerald 0 Color centers 0 Another way for a gem to absorb specific light wavelengths is a color center 0 Color centers are imperfections in the lattice of the crystal 0 Imperfections can be due to a number of factors I Excess of one element I Lack of another element I Substitutional impurities different ions that quotpushquot their way into the lattice I Mechanical deformation of the crystal lattice I Can be caused by heat pressure or even radiation 0 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 0 Inclusions color invaders 0 Color can arise from bits of other minerals inclusions that are incorporated within the gem39s crystal many quartz varieties get their color this way I Chalcedony varieties jasper fuschite and aventurine all get their color from inclusions of other minerals III Variation in Color 0 Minerals don39t always have constant color throughout either 0 Many times color is irregularly distributed within the gem creating a somewhat patchy appearance 0 Sometimes the growth pattern of a crystal will create distinct bands of color 0 As the gem crystal grows specific bands or areas of color will form I Some color patterns can be diagnostic especially for nontransparent minerals 0 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 0 Tourmaline O Tourmaline exhibits some fascinating color variation I Some varieties of tourmaline vary along the length of the crystal 0 Varying from green at the base to colorless to red at the top I There are also color variations that occur from the inside out red in the middle green on the outside 0 This variation has to do with the growth of the crystal and the presence of certain transition metals during phases in the crystal39s formation 0 Quartz 0 Ametrine is a bicolored quartz with amethyst and citrine on either end I Amethyst will show distinctive bands of color that follow the crystal growth lines 0 This is diagnostic an evenly colored amethyst is probably either synthetic or not amethyst at all 0 Malachite 0 Malachite shows variation in the shade of green that it exhibits I This is due to the different crystal sizes I Smaller crystals re ect light differently and look lighter than larger crystal bands IV Color Enhancement Efforts to improve the color of a gem address many of these issues 0 There are many ways to alter the color of a gem 0 Before the common method was to dye or apply colored oil to the surface of a stone I Other more complicated procedures involve radiation andor heat to alter the color centers within a stone I Sometimes high pressure can change the crystal lattice and alter the color quotOilingquot is a common method of gem enhancement 0 A thin layer of oil inside the cracks of a gemstone even out the re ected light and make the gem look more brilliant and also improve the clarity I The light re ected in all directions on the left makes the surface appear dull but the light re ecting 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 I EX sapphires are routinely heat treated the high temperatures dissolve rutile inclusions within the gem 0 This serves two purposes it makes more titanium ions available to act as chromophores and improves the clarity by dispersing the inclusion
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