Webster's English Dictionary

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spinel

spi.nel or spi.nelle \sp*-'nel\ n [It spinella, dim. of spina thorn, fr. L] 
   1: a hard crystalline mineral MgAl2O4 consisting of an oxide of  magnesium 
   and aluminum that varies from colorless to ruby-red to black and is used as 
   a gem 2: any of a group of minerals that are essentially oxides of 
   magnesium, fer rous iron, zinc, or manganese