Colored Stones 23 - Tourmaline, Peridot, Zircon PDF

Colored Stones 23 KEY CONCEPTS Tourmalines are a group of minerals that share the same Paraíba tourmaline’s high prices result from a combination of crystal structure but have slightly different chemical and attractive, highly saturated hues, extremely restricted supply, physical p...

Colored Stones 23 KEY CONCEPTS Tourmalines are a group of minerals that share the same Paraíba tourmaline’s high prices result from a combination of crystal structure but have slightly different chemical and attractive, highly saturated hues, extremely restricted supply, physical properties. and strong demand. Most gem tourmalines are elbaites that form in pegmatites. Heating and irradiation are the two most important tourmaline treatments. Some well-formed, fine-color tourmaline crystals are more valuable as mineral specimens than as fashioned gems. Because peridot is sensitive to rapid temperature changes and chemicals, it’s best cleaned with warm water and mild The shapes of many tourmaline crystals often force cutters to detergent or soap. fashion gems with slender proportions. Sources in Myanmar supply the world with most of its large, Because of tourmaline’s strong pleochroism, cutters have to fine-color peridot. orient finished gems carefully to achieve the best colors. Arizona supplies the world with rough for much of its commercial calibrated peridot. Brazil supplies the majority of the world’s gem-quality tourmaline. Colorless zircon’s strong double refraction easily distinguishes it from diamond and synthetic cubic zirconia. Nigeria and Mozambique are Africa’s most economically important tourmaline sources. Because of its brittleness, zircon needs care in handling and wear. Traces of copper cause Paraíba tourmaline’s extraordinary colors. Virtually all blue and colorless zircons have been treated. Key Terms Chrome tourmaline—Trade term for a vibrant green Parti-colored—Gems that show color zoning with two or tourmaline that’s colored by traces of vanadium, more colors. chromium, or both. Rubellite—Trade name for pink to red elbaite tourmaline. Indicolite—Trade term for blue tourmaline. Watermelon tourmaline—A tourmaline, usually elbaite, Metamict—A mineral that’s been broken down from a with color zoning made up of a pink central zone crystalline to a partly amorphous state by emanations surrounded by a green overgrowth. from radioactive impurities. Paraíba tourmaline—Trade term for rare, expensive, vibrant green to blue or violet elbaite tourmaline, originally found only in Brazil’s Paraíba state. 102