Tourmaline

 
Color : Colorless, pink, red, yellow, brown, violet, black, multi-colored   
Color of streak: White

Mohs’ hardness : 7-7.5
Specific gravity : 3.02-3.26   
Cleavage : None 

Fracture: Uneven, small conchoidal, brittle
Crystal system : Hexagonal
Chemical composition : aluminium borate silicate, complicated and changeable composition
Refractive index : 1.616-1.652
Double refraction: -0.014 to –0.044 
Fluorescence:
Colorless, weak ; green-blue, pale yellow : weak ; green-blue, red : weak ; red-violet, pink, brown, green, blue: none 

No gemstone has such richness in color variation as tourmaline. Known in antiquity in the Mediterranean area, the Dutch imported it in 1703 from Sri Lanka into Europe. They gave the new stone a Sinhalese name Turamali, the original meaning of which is not know. 

Uni-colored tourmalines are quite rare. Most crystals have various color shades or even different colors. Often there is some layered color. Brazil produces stones with a red interior, inner “skin” white, outer “skin” green. The South African tourmalines are green inside and the outer layer is red. A tourmaline with a red inside and a green “skin” is sometimes called a “watermelon”. The nuances and colors are particularly effectively shown when slices of cross-section are polished.

The names for the colored tourmalines include indicolite (blue in all shades), paraiba (turquoise - the highest priced of the colors), rubellite (shocking pink), dravite (brown), siberite (lilac to violet blue), achroite (white), verdelite (green in all shades and the emerald green is most valuable among the green ones).

Tourmaline cat’s eyes exist in various colors, but only in the pink and green varieties is the chatoyancystrong, caused by inclusions of foreign crystals. Some tourmalines show a slight change of color inartificial light.