samarskite

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A velvet-black mineral that is a complex mixture of several rare-earth metals with niobium and tantalum oxide.

from The Century Dictionary.

• noun A niobate of uranium, iron, and manganese, of a velvet-black color, submetallic luster, and conchoidal fracture.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Min.) A rare mineral having a velvet-black color and submetallic luster. It is a niobate of uranium, iron, and the yttrium and cerium metals.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mineralogy A shiny black radioactive mineral that contains a complex mixture of rare earth oxides

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a complex black mineral occurring in pegmatites