spinel

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A hard, variously colored mineral with composition MgAl₂O₄, having usually octahedral crystals and occurring in igneous and carbonate rocks. The red variety is valued as a gem and is sometimes confused with ruby.
2. n. Any of a group of minerals that are oxides of magnesium, iron, zinc, manganese, or aluminum.

Century Dictionary and Cyclopedia

1. n. A mineral of various shades of red, also blue, green, yellow, brown, and black, commonly occurring in isometric octahedrons. It has the hardness of topaz. Chemically, it consists of the oxids of magnesium and aluminium, with iron protoxid in some varieties, also chromium in the variety picotite. Clear and finely colored red varieties are highly prized as ornamental stones in jewelry. The red varieties are known as spinel ruby or balas ruby, while those of a dark-green, brown, or black color, containing iron protoxid in considerable amount, are called ceylonite or pleonaste. The valuable varieties, including the spinel ruby (see ruby), occur as rolled pebbles in river-channels in Ceylon, Burma, and Siam; they are often associated with the true ruby (corundum). The spinel group of minerals includes several species which may be considered as made up of equal parts of a protoxid and a sesquioxid (RO + R₂O₃). Here belong gahnite, magnetite, franklinite, etc. An octahedral habit characterizes them all.
2. n. A bleached yarn from which the linen tape called inkle is made.

Wiktionary

1. n. mineralogy any of several hard minerals of cubic symmetry that are mixed oxides of magnesium and aluminium and are used as gemstones of various colours
2. n. solid state chemistry any crystalline material, not necessarily an oxide, that possesses the same crystal structure as this mineral
3. n. Bleached yarn in making the linen tape called inkle; unwrought inkle

GNU Webster's 1913

1. n. (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.
2. n. Bleached yarn in making the linen tape called inkle; unwrought inkle.

WordNet 3.0

1. n. a hard glassy mineral consisting of an oxide of magnesium and aluminum; occurs in various colors that are used as gemstones