The Collaborative International Dictionary of English v.0.48:

Nugget \Nug"get\, n. [Earlier niggot, prob. for nigot, an ingot.
   See Ingot.]
   1. A lump; a mass, esp. a native lump of a precious metal;
      as, a nugget of gold.
      [1913 Webster]

   2. Something of value, usually found among many other less
      valuable things; -- often used figuratively; as, a nugget
      of useful information in a sea of data.
      [PJC]

WordNet (r) 3.0 (2006):

nugget
    n 1: a solid lump of a precious metal (especially gold) as found
         in the earth

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "nugget":
   bar, batch, block, bulk, bullion, chunk, clod, clump, coin gold,
   coin silver, copper, gob, gold, gold nugget, hunch, hunk, ingot,
   loaf, lump, mass, nickel, pat, precious metals, quantity, silver,
   wad, yellow stuff