Source : Webster's Revised Unabridged Dictionary (1913)

Basalt \Ba*salt"\, n. [N. basaltes (an African word), a dark and
   hard species of marble found in Ethiopia: cf. F. basalte.]
   1. (Geol.) A rock of igneous origin, consisting of augite and
      triclinic feldspar, with grains of magnetic or titanic
      iron, and also bottle-green particles of olivine
      frequently disseminated.

   Note: It is usually of a greenish black color, or of some
         dull brown shade, or black. It constitutes immense beds
         in some regions, and also occurs in veins or dikes
         cutting through other rocks. It has often a prismatic
         structure as at the Giant's Causeway, in Ireland, where
         the columns are as regular as if the work of art. It is
         a very tough and heavy rock, and is one of the best
         materials for macadamizing roads.

   2. An imitation, in pottery, of natural basalt; a kind of
      black porcelain.

Source : WordNet®

basalt
     n : the commonest type of solidified lava; a dense dark gray
         fine-grained igneous rock composed chiefly of
         calcium-rich plagioclase feldspar and pyroxene