The Collaborative International Dictionary of English v.0.48:

Diatomous \Di*at"o*mous\, a. [Gr. dia`tomos cut through, fr.
   diate`mnein to cut through; dia` through + te`mnein to cut.
   Cf. Diatom.] (Min.)
   Having a single, distinct, diagonal cleavage; -- said of
   crystals. --Mohs.
   [1913 Webster]