A textural term for rocks and minerals made of a mosaic of microscopic crystals. Unlike amorphous materials like glass, cryptocrystalline substances have a true crystalline structure, but the individual grains are so fine that they cannot be distinguished with a hand lens. Chert, flint, and chalcedony are classic examples.
Heard on the job
The chert band is entirely cryptocrystalline, so you won't see any cleavage faces with your loupe.
Insider tip
If it breaks with a smooth, conchoidal fracture and looks a bit like dull glass, it is likely cryptocrystalline.
Do not mix it up with
- Aphanitic: Aphanitic crystals are invisible to the naked eye but often visible under a hand lens, whereas cryptocrystalline grains require a microscope.