The Collaborative International Dictionary of English v.0.48:

Cryptocrystalline \Cryp`to*crys"tal*line\ (-kr?s"tal-l?n), a.
   [Gr. krypto`s hidden + E. crystalline.] (Geol.)
   Indistinctly crystalline; -- applied to rocks and minerals,
   whose state of aggregation is so fine that no distinct
   particles are visible, even under the microscope.
   [1913 Webster]