Definition of spinels:
(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.
(n) :
(solid state chemistry) Any crystalline material, not necessarily an oxide, that possesses the same crystal structure as this mineral.
(n) :
(obsolete) Spindle; the amount of yarn that fits on a spindle.