crystallizes in the orthorhombic system and is in composition a fluosilicate of aluminium, its hardness is about 8. It is transparent with the colours yellow, reddish-brown, blue, rose-red, green or colourless.

Sources of  Topaz: Russia, Brazil, Sri Lanka, and Burma. 
 
 

Despite of its hardness, Tapaz is very brittle and therefore very difficult to cut. 
Blue Topaz is most  longing, that´s why colourless or miscoloured gem material is often irradiated to reach a rich blue colour.