Spinel

Spinel is the magnesium aluminium member of
the larger spinel group of minerals. It has the
formula MgAl2O4 in the cubic crystal system.
Its name comes from Latin "spina" (arrow).
Though spinels are often referred to as rubies, as
in the Black Prince Ruby, the true ruby is not a
spinel. Balas ruby is an old name for a rose-tinted
variety of spinel.