Spinel

 
Color : Red, pink, violet, yellow, orange, blue, dark green, black  
Color of streak: White

Mohs’ hardness : 8
Specific gravity : 3.58-3.61   
Cleavage : Imperfect 

Fracture: Conchoidal, uneven
Crystal system : Isometric
Chemical composition : Magnesium aluminium oxide
Refractive index : 1.712-1.736
Double refraction : none
Fluorescence : Red : Strong ; red ; blue : weak ; reddish or green ; green ; weak ; reddish 


The derivation of the name “spinel” is uncertain. It may mean “spark” (Greek) or “point” (Latin). Spinel occurs in all colors, the favourite being a ruby-like red. The pigments are chrome and iron. Large stones are rare and star spinels very rare.

The blue variety is sensitive to high temperatures. The dark green to black opague spinel is called Ceylonite, also pleonaste (Greek – surfeit, because of its numerous crystal faces). The brown variety is called Piconite (French) yellow is known as Rubicelle (diminutive of French word for ruby) and the pale red is known as balas ruby (after a region of Afganistan).

Spinel was recognized as an individual mineral only 150 years ago. Before then it was classed as ruby, because it also occurs with it. Some well-known “rubies” are really spinels, such as the 1.9in / 5cm long, oval “Black Prince’s Ruby” also the 361ct “Timur Ruby” in a diamond set necklace; both are in the English Crown Jewels. Both are uncut and only polished. The drop shaped spinels in the Wittelsbacher’s crown of 1830 were also originally thought to be rubies.