The Collaborative International Dictionary of English v.0.48:

Nicker nut \Nick"er nut`\
   A rounded seed, rather smaller than a nutmeg, having a hard
   smooth shell, and a yellowish or bluish color. The seeds grow
   in the prickly pods of tropical, woody climbers of the genus
   Caesalpinia. Caesalpinia Bonduc has yellowish seeds;
   Caesalpinia Bonducella, bluish gray. [Spelt also neckar
   nut, nickar nut.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

nicker nut
    n 1: hard shiny grey seed of a bonduc tree; used for making e.g.
         jewelry [syn: bonduc nut, nicker nut, nicker seed]