chiastolite

from The Collaborative International Dictionary of English v.0.48

Chiastolite \Chi*as"to*lite\ (k[-i]*[a^]s"t[-o]*l[imac]t), n.
   [Gr. chiasto`s marked with a [chi] + -lite. See {Chiasm}. So
   called from the resemblance of the cross cuts of its crystals
   to the Greek letter [chi].] (Min.)
   A variety of andalusite; -- called also {macle}. The
   tessellated appearance of a cross section is due to the
   symmetrical arrangement of impurities in the crystal.
   [1913 Webster]