Source : Webster's Revised Unabridged Dictionary (1913)

Fergusonite \Fer"gu*son*ite\, n. (Min.)
   A mineral of a brownish black color, essentially a
   tantalo-niobate of yttrium, erbium, and cerium; -- so called
   after Robert Ferguson.

Source : WordNet®

fergusonite
     n : a dark mineral consisting of oxides of yttrium and erbium
         and tantalum and other minerals