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 opaque 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 Rucicelle (diminutive of French word for ruby) and the pale red is known as balas ruby (after a region of Afghanistan)
Spinel was recognized as an individual mineral only 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” and also 361ct “Timur ruby” ia a diamond set necklace; both are in the English Crown jewels . Both are uncut and only polished. The drop-shaped spinels in the Witterbacher’s crown of 1830 were also originally thought to be rubies.
The 2 largest spinels (formed as roundish octahedrons) weigh 520ct and each are in the British Geological Museum in London.

*Info by Walter Schumann “Gemstones of the World”