The Collaborative International Dictionary of English v.0.48:

Spinel \Spi*nel"\, Spinelle \Spi*nelle"\, 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]

   Note: 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]