Spinel is a mineral composed chiefly of magnesia and alumina, and crystallising in octahedra. There are several varieties, which differ in chemical composition owing to isomorphous substitution, sometimes of the protoxides, but usually of the sesquioxides. Ruby or Magnesia Spinel is clear red or reddish, and contains little or no iron;
Pleonaste (Ceylonite), or Iron-magnesia Spinel, is dark green to black, containing iron; Picotite, or Chrome Spinel, is black, containing chromium oxide; Gahnite, or Zinc Spinel, is green to brown, containing zinc; Hercynite, or Iron Spinel, is black, and occurs massive, in this variety ferrous oxide replacing magnesia. Ruby spinel occurs as crystals imbedded in granular limestone, also with calcite in serpentine and in various crystalline schists, as also in the alluvial sands, &c. derived from the disintegration of these rocks. Pleonaste is an occasional constituent of eruptive rocks, and also of certain rocks which have undergone alterations from contact with eruptive masses. Picotite has been met with in basalt, but is more commonly seen in peridotite. Gahnite is of sparing occurrence in certain crystalline schists, while Hercynite is occasionally found in such rocks as granulite. The finer varieties of spinel are prized as gems—the red coloured ones being commonly called rubies. See RUBY.