Gemstones Week of April 14
Gemstones Week of April 14 1108
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This 4 page Class Notes was uploaded by Caroline Bacevice on Friday April 17, 2015. The Class Notes belongs to 1108 at Ohio State University taught by Loren Babcock in Spring2015. Since its upload, it has received 36 views. For similar materials see Gemstones in Earth Sciences at Ohio State University.
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Date Created: 04/17/15
Gemstones Week of April 14 Origin of Quarts Common in most igneous sedimentary and metamorphic rock Quartz is quite resistant to mechanical abrasion it is the most abundant mineral following weathering and erosion 0 Common in stream sediments on beaches in windblown deposits Synthetic quartz First synthesized in the 18005 by the hydrothermal method Heat Treatment of Quartz Formation of citrine from amethyst is common 0 Often done by using a wheelbarrow to immerse crystals in sand within a fire 0 Crystals are heated to a high enough temperature to change the oxidation state of iron in the mineral Piezoelectricity little electricity What is piezoelectricity o A few crystal types have polar axes instead of a symmetry center 0 The opposite axes have different properties 0 Pressure exerted at the ends of a polar axis causes electrons to ow to one end producing a negative charge and a positive charge is induced at the other end Quartz and tourmaline are strongly piezoelectric Plates cut from quartz have been used as oscillators to control radio frequency 0 When subjected to alternating current of a radio circuit quartz vibrates at a frequency that depends on plate thickness and type of cut Tiny quartz plates in watches vibrate at a constant predetermined frequency to control the radio frequency of an electric circuit that provides the time display 0 Synthetic quartz is used today for this purpose Chalcedony and fine grained quartz Quartz Translucent to Opaque Varieties Chalcedony Cryptocrystalline quartz o Agate Banded all colors 0 Petrified Wood wood fossilized by quartz o Crysoprase Green Bloodstone Dark green with red spots Iasper All colors often reddish brown Carnelian Brownish red to orange Moss Agate Colorless with green brown or red inclusions of hornblende or chlorite in mosslike patterns 0 Onyx Layered commonly black base and white upper layer 0 Sard red brown to brown Flint Fine grained quartz Tiger s eye quartz Chatoyant with asbestos needles gold yellow and gold brown OOOO Origin of name From Sanskrit upala stone or precious stonequot Composition Amorphous noncrystaline hydros silica SiOanHZo Hardness 565 Colors all colors Streak white Other Characteristics Play of colors may be present sensitive to bumping acids and alkalines Types of Opal Precious opal has distinctive play of colors opalescence 0 Play of colors due to microscopic spheres of cristobalite silica layered in siliceous gel 0 Cristobalite spheres cause diffraction and interference patterns Fire Opal yellowred or orange color often without play of color Transparent best to translucent Common opal Opaque rarely translucent without play of colors Water in Opal Treatments Opal always contains some water 0 Typically 330 water In time the stone may lose water causing cracking and reduction of opalescence 0 Can be restored temporarily by saturation with oil epoxy resin or water 0 Storing opal in moist cotton minimized dehydration Care is needed in mounting opal in jewelry Heat used in soldering can evaporate the water Opal is often impregnated with plastic to improve its appearance Often mounted with quartz glass or spinel cap doublet Obsidian Origin of name After Roman Obsius Composition Amorphous silica glass mostly SioZ Hardness 555 Colors black grown gray green Streak white Specific gravity 24 Other characteristic Conchoidal fracture important for making Occurrences Chalcedony and fine grained quartz occurs in igneous and sedimentary rocks worldwide Opal occurs in sedimentary rocks in many areas of the world 0 Major sources Australia Mexico Obsidian Occurs in igneous rocks in many parts of the world 0 Major sources SW USA
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