Colored Stones 2 03 05 2014 Luminescence emission of light from a gem after exposure to UV radiation Fluorescence Phosphorescence Pleochroism Color change Iridescence play of colors Topaz Zircon Chrysoberyl Spinel Of what consequence is radioactivity in minerals Topaz Origin and history of name Topaz Possibly after the island of Topazos in the Red Sea now called Zabargad Egypt o Topazos was the ancient source of peridot olivine In ancient times all yellow and many green and golden brown gems were called topaz Sometimes called precious topaz or imperial topaz to distinguish it from quartz varieties lemon topaz citrine smoky topaz quartz Topaz Characteristics Composition Aluminum Fluorohydroxysilicate SILICATE 8 Hardness All colors White Streak SG 3 53 RI 1 62 1 63 DR Orthorhombic Crystal System Vitreous Luster Pleochrosim Usually yellow in yellow topaz or pink in red topaz Origin of topaz Formed as a fluorine bearing vapors are given off in the last stages of solidification of igneous rocks o Occurs in cavities in rhyolite and granite especially in pegmatites o Secondary source pebbles in stream gravels Sources Brazil Russia Japan Mexico Sri Lanka Burma Utah Synthesis and enhancements Blue topaz has been synthesized since 1976 Almost all blue topaz sold today is produced by irradiating and then heating natural colorless topaz Topaz is the most commonly irradiated gem on the market today However irradiation is used to change the color of many gems o Why does it matter Radiation changes the natural color of the gem reducing its value Irradiation cannot be detected Color is not always stable Safety residual radioactivity to be stored to allow radioactivity to decay half life may be hours to days Zircon Origin and history of name Zircon origin of name is uncertain Possibly from Arabic Zargun derived from Persian Zar and gun gold colored Zircon Characteristics Composition Zirconium Silicate Hardness 6 7 5 Colors Colorless yellow brown orange red violet blue green White streak SG 4 7 high 3 95 low due to radiation damage metamict RI 1 81 1 98 DR Tetragonal crystal system Luster resinous to adamantine Origin of Zircon Zircons are usually found in gravel deposits o Zircon originally forms as small crystals in igneous and metamorphic rocks but these sources are not mined o Zircon is highly resistant to weathering and has a high SG both of which factor its concentration in alluvial gravel s Most important localities Laos Cambodia Thailand Important Characteristics of Zircon Double refraction of crystals causing obvious doubling of facets Contains uranium and thorium both of which undergo radioactive decay to lead metamict o Crystals become damaged cloudy with radioactive decay o Zircons are used for isotopic age dating Radioactivity Dosage Zircon 1 nanoCurie nCi grm of U238 o nCi billionth of a Curie o Curie radioactivity in 1 g of radium Irradiated blue topaz 25 nCi g Average human body contains 200 nCi of radioactive potassium K40 Average human consumes 140 nCi of K40 per year Radioactivity dosage from gems is insignificant Chrysoberyl 03 05 2014 Chrysoberyl From Greek chrysos golden and beryllos referring to beryl Golden beryl a misnomer Alexandrite variety showing color change in different types of light named for Czar Alexander II of Russia o Very commonly imitated Cymophane From Greek Cymos waving and phanos light Chrysoberyl Characteristics Composition Beryllium aluminum oxide Hardness 8 5 Colors Yellow green yellow green brownish red light blue Streak White SG 3 73 RI 1 75 1 76 Crystals Orthorhombic Luster Vitreous Origin Occurs in igneous rocks granites pegmatites and meramorphic rocks mica schists Also occurs in alluvial deposits of Sri Lanks Brazil China India Alexandrite Discovered in the Ural Mountains Russia in 1833 Now found in gem gravels of Sri Lanka mined in Zimbabwe Brazil Myanmar Zambia India Synthesized by flame fusion Czochralski process crystal pole Much lab created Alexandrite is beryllium doped corundum method sapphire Chatoyancy Silky sheen resulting from closely packed parallel fibers or parallel needlelike inclusions Produces cat s eye effect when stones are cut in cabochon Chatoyancy is present in various minerals chrysoberyl quartz o Chatoyancy Chrysoberyl is called Cymophane o Tiger s eye Quartz replacement of fibrous crocidolite asbestos Spinel 03 05 2014 Spinel Origin and history of name Spinel From Latin spina little thorn spine referring to sharp points on octahedral crystals Red spinel has often been confused with ruby o Many historic rubies were probably spinel Spinel Characteristics Composition Magnesium aluminum oxide similar to corundum and chrysoberyl but it has magnesium Hardness 8 Colors Colorless to all usually red allochromatic just like corundum Streak white SG 3 6 RI 1 72 SR Crystals Cubic often octahedron Vitreous luster Origin of Spinel Igneous and metamorphic mineral Gem quality spinel usually forms as a contact metamorphic mineral in limestone Also found as rolled pebbles in sands and gravels Gem gravels of Burma Sri Lanka Thailand Synthesize by flame fusion verneuil Famous Spinel Black Prince s Ruby British imperial crown of state Timor Ruby british crown jewels o Actually spinel Rock Forming Minerals 03 05 2014 What are rock forming minerals Quartz Olivine Pyroxene Amphibole Feldspar Tourmaline Garnet Zoisite Quartz Transparent Varieties Rock Crystal Colorless Amethyst Purple Citrine Yellow Smoky Quartz Rose Pink Ametrine Color zoned amethyst and citrine Aventurine dark green with glittery mica inclusions Rutilated quartz colorless with rutile inclusions Prasiolite leek green Quartz cat s eye chatoyant various crystals Quartz Translucent to opaque varieties Various forms of cryptocrystalline and fine grained quartz including o Chaledony including agate Agate Banded all colors Petrified Wood wood fossilized by quartz Chrysoprase green Bloodstone dark gree with red spots Jasper all colors often reddish brown Carnelian brownish red to orange Moss agate colorless with green brown or red inclusions of hornblende or chlorite in moss like patterns Onyx layered commonly black base and white upper layer Sard red brown to brown o Flint Chert official state gemstone of ohio Quartz Origin and history of some names Quartz from german quartz originally applied to massive vein Rock Crystal From Greek krystallos ice Amethyst From greek amethystos not to intoxicate Citrine From old french citron yellow Chalcedony possibly from chalcedon an ancient