Spinel (Balas Ruby is an old name for the rose-tinted variety) is the magnesium aluminium member of the larger spinel group of minerals; which also includes iron and chromium spinels.
Spinel may be colourless, but usually offers a range of hues, from orange to intense "stoplight" red, vibrant pink, and all shades of purple, blue, and violet through bluish green, yellow-brown or black. It crystallizes in the isometric system; common crystal forms are octahedra, usually twinned and has an imperfect octahedral cleavage and a conchoidal fracture.
There is a unique natural white spinel, now lost, that surfaced in Sri-Lanka. Intense reds and pinks are caused by traces of chromium. The higher the chromium content, the stronger the red hue. Orange and purple stones owe their colour to a mixture of iron and chromium. Violet to blue spinel can be coloured by trace amounts of iron, and vibrant blues owe their saturated colour to trace amounts of cobalt.
Some spinels are among the most famous gemstones, such as the Black Prince's Ruby, the "Timur Ruby" in the British Crown Jewels and the "cote de Bretagne", formerly from the French Crown Jewels.
The Samarian Spinel is the largest known in the world, weighing 500 carats (100g).
The transparent red spinels were called spinel-rubies or balas rubies. In the past, before the arrival of modern science, spinels and rubies were equally known as rubies.
After the 18th centuary the word ruby was only used for the red gem variety of the mineral corundum and the word spinel became used. "Balas" is derived from "Balascia", the ancient name for Badakhshan, a region in central Asia. The Badakshan Province was for centuries the main source for pink and red spinels.