Source : Webster's Revised Unabridged Dictionary (1913)

Nugget \Nug"get\, n. [Earlier niggot, prob. for nigot, an ingot.
   See {Ingot}.]
   A lump; a mass, esp. a native lump of a precious metal; as, a
   nugget of gold.

Source : WordNet®

nugget
     n : a solid lump of a precious metal (especially gold) as found
         in the earth