Spinel ( /spɪˈnɛl/) 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 ruby is not a spinel. Balas ruby is an old name for a rose-tinted variety of spinel.