conchoidal. Hardness = 8. Specific gravity = 
,
and for gem varieties it is 
The
gem quality spinels have a wide range of colours such as crimson, pink,
yellow, brown, green, blue, purple, violet and black. The red and green
varieties are thought to have been coloured by chromium, and the blue
variety by cobalt. Lustre: Vitreous, splendent to dull and the mineral
varies from transparent to opaque. Refractive index = 
Dispersion = 
Isotropic.
The general chemical formula may be written as 
; M stands for aluminium, ferric iron, manganese or chromium. Such a
combination yields quite a wide range of minerals. Spinel proper is 
The magnesia may in part be replaced by ferrous iron, zinc, manganese or cobalt, and the ferric iron by chromium.
Uses
The
spinels form such transparent and sparkling crystals, that, when they
have the ruby colour, they may be even sold as a ruby. Good gem spinel
also fetches a decent price. Good crystals of spinel without colour are
not much in demand.
Grades of Varieties
1. Ruby spinel or spinel ruby.—It is deep red and transparent, the most popular gem variety, also known in India as Lai runumi or the 'Pomegranate ruby'.
2. Balas ruby.—Rose
red to pink, has the same composition as the spinel. The Greeks
connected 'Balas' with 'Palatius' as they supposed it to be the matrix
in which the ruby was found.
3- Rubicelle.—Yellow to orange red.
4. Almandine.—Violet and purple.
5. Sapphirine.—Blue spinel.
6. Chlorospinel.—Iron bearing and of a grass-green colour.
117
