spinel

{ } n. [ F. spinelle, or LL. spinellus, perhaps from L. spina a thorn, a prickle, in allusion to its pointed crystals. ] (Min.) A mineral occuring in octahedrons of great hardness and various colors, as red, green, blue, brown, and black, the red variety being the gem spinel ruby. It consist essentially of alumina and magnesia, but commonly contains iron and sometimes also chromium. [ 1913 Webster ]

☞ The spinel group includes spinel proper, also magnetite, chromite, franklinite, gahnite, etc., all of which may be regarded as composed of a sesquioxide and a protoxide in equal proportions. [ 1913 Webster ]