Pearlite

from WordNet (r) 3.0 (2006)

pearlite
    n 1: a lamellar mixture of cementite and ferrite formed during
         the cooling of austenite; a constituent of steel and cast
         iron

from The Collaborative International Dictionary of English v.0.48

Pearlite \Pearl"ite\, Pearlstone \Pearl"stone`\, n. (Min.)
   A glassy volcanic rock of a grayish color and pearly luster,
   often having a spherulitic concretionary structure due to the
   curved cracks produced by contraction in cooling. See Illust.
   under {Perlitic}.
   [1913 Webster]