TopazTopaz

Transparency: Transparent, translucent

Refractive index: 1.609-1.643

Double refraction: +0.008 to +0.016

Dispersion: 0.014 (0.008)

Pleochroism: Yellow: definite; lemon-honey, straw-yellow Blue: weak; light and dark blue, Red: strong; dark red, yellow, pink-red

Absorption spectrum: Pink: 682

Fluorescence: Pink: weak; brown Red: weak; yellow-brown Yellow: weak; orange-yellow

Colour: Colourless, yellow, orange, red-brown, light to dark blue, pink-red, red, violet, light green

Colour of streak: White

Mohs' hardness: 8

Density: 3.49-3.57

Cleavage: Perfect

Fracture: Conchoidal, uneven

Crystal system: Orthorhombic, prisms with multi-faceted ends, often 8-sided in cross-section striations along length

Chemical composition: AI2SiO4(F,OH)2 fluor containing aluminium silicate
   Formerly, the name topaz was not applied consistently or specifically; one called all yellow and golden-brown, and sometimes also green, gemstones topaz. The name topaz is most probably derived from an island in the Red Sea, now Zabargad but formerly Topazos, the ancient source of peridot. Colours of the gemstone that is today called topaz are rarely vivid. The most common colour is yellow with a red tint; the most valuable is pink to reddish-orange. The colouring agents are iron and chromium. Some yellowish-brown varieties of certain deposits gradually fade in the sunlight. Care must be taken during polishing and setting because of the danger of cleavage. They are also not resistant to hot sulphuric acid. The lustre is vitreous. Deposits are associated with pegmatites or secondary placers. During the 18th century, the most famous topaz mine was at Schneckenstein in the southern Voigtland in Saxony. Today, Brazil (Minas Gerais) is the most important supplier. Other deposits are in Afghanistan, Australia, Burma (Myanmar), China, Japan, Madagascar, Mexico, Namibia, Nigeria, Pakistan, Russia (the Urals, Transbaikalia), Zimbabwe, Sri Lanka, and the United States. Light blue topazes are found also in Northern Ireland, Scotland, and Cornwall, England. Topazes weighing several pounds are known. In 1964 some blue topazes were found in the Ukraine, each weighing about 220 lb (100 kg). The Smithsonian Institution in Washington, D.C., owns cut topazes of several thousand carats each. Coloured stones are usually step- (emerald) or scissor-cut, and colourless ones or weakly coloured ones are brilliant-cut. Topazes with disordered inclusions are cut en cabochon. Possibilities for Confusion With apatite , aquamarine , brazil-ianite , chrysoberyl , citrine , danburite , diamond , precious beryl , fluorite , kunzite , orthoclase , phenakite , ruby , sapphire , spinel , tourmaline , and zircon . Since 1976, blue synthetic topazes are known. Almost all blue topaz sold today is produced by first irradiating and then heating natural colourless topaz. Since the quartz variety citrine is in the trade often falsely called "gold topaz" or "Madeira topaz," real topaz is sometimes called precious topaz, in order to clearly distinguish them.