Topaz is a form of aluminum silicate fluoride hydroxide. Because of strong chemical bonds within this mineral, topaz is the hardest of silicate minerals.
This gem, with its lively fire, clarity, beautiful colors and hardness is ideal for jewelry such as clips, necklaces, brooches and bracelets. Pure topaz, when brilliantly cut, can be often mistaken for a diamond. Because of its rarity, topaz is an expensive gem. The most valued and rarest color is red. Imperial topaz -- sherry colored varieties of brownish-yellow, orangish yellow and reddish brown -- are the most popular topaz stones and command high prices, as do pink colored stones. Light blue and pale yellow topaz are of less value, but are nevertheless stunning in beauty.
•Topaz Chemical composition: Al2SiO4(F,OH)2
•Class: nesosilicate
•Crystal system: orthorhombic; 2/m2/m2/m
•Crystal habit: commonly as euhedral prismatic crystals terminated by dipyramids, first and second order prisms, and basal pinacoids. Frequently with vertical striations on the prismatic faces. Also appears in crystalline masses.
•Twinning: twins are very rare in topaz
•Specific gravity: 3.4 - 3.6
•Index of refraction: 1.606 - 1.638
•Birefringence: weak (0.008-0.011)
•Pleochroism: weak to moderate, tones vary depending on color of specimen
•Hardness: 8
•Color: commonly colorless, also light to medium blue, yellow/orange/pink/red/violet with or without brown tone, brownish green, brown.
•Luster: vitreous
•Transparency: transparent to translucent
•Cleavage:perfect on {001}
•Fracture: conchoidal, brittle
•Streak: white
•Fluorescence: mild flourescence in some crystals