samarskite

American Heritage® Dictionary of the English Language, Fourth Edition

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

Century Dictionary and Cyclopedia

1. n. A niobate of uranium, iron, and manganese, of a velvet-black color, submetallic luster, and conchoidal fracture. It is found in the Ilmen mountains, also in considerable quantity in North Carolina. It has yielded a number of new elements, belonging especially to the yttrium group (decipium, philippium, etc.), whose properties are not as yet wholly determined.

Wiktionary

1. n. mineralogy A shiny black radioactive mineral that contains a complex mixture of rare earth oxides

GNU Webster's 1913

1. adj. (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.

WordNet 3.0

1. n. a complex black mineral occurring in pegmatites