Topaz

 
Color : Colorless, yellow, red-brown, light blue, pinky red, pale green  
Color of streak: White

Mohs’ hardness : 8
Specific gravity : 3.53-3.56   
Cleavage : Perfect 

Fracture: Conchoidal, uneven
Crystal system : Orthorhombic
Chemical composition : fluor containing aluminium silicate
Refractive index : 1.610-1.638
Double refraction: +0.008-+0.010
Fluorescence: Pink : weak ; brown, red : weak ; yellow-brown, yellow : weak ; orange-yellow


In antiquity all yellow and brown gemstones, even green ones, were called “topaz”. The name most probably derived from the name of an island in the Red Sea, now Zebirget, formarly Topazos.

Colored stones are rarely vivid. The most common color is yellow with a red tint. The most valuable is pink. Siberian topazes lose some color in sunlught. The rough stones should not be scratch tested because of the danger of cleavage. Care must be taken during polishing and setting for the same reason. They are not resistant to sulphuric acid.