cryptocrystalline

Cryptocrystalline describes a texture in which the crystals that compose a mineral are so small that they cannot be resolved with the naked eye or a standard hand lens. The crystals are typically intergrown at submillimeter or micrometer scales, giving a smooth or waxy appearance without visible crystal faces. In rocks and minerals, cryptocrystalline textures contrast with macrocrystalline textures, where crystals are large enough to be seen clearly.

The term is commonly applied to silica-rich materials such as chalcedony, agate, jasper, and chert. These materials consist of intergrowths of quartz crystals that are too small to be distinguished individually, resulting in a uniform microtexture rather than distinct crystals. Cryptocrystalline silica can form through depositional processes in sedimentary environments or as replacements of preexisting rocks, often filling voids in volcanic or sedimentary settings and producing banded varieties (as in agate) or opaque forms (as in jasper).

Physical properties reflect the quartz family most often involved: hardness around 6.5–7 on the Mohs scale, a waxy to dull luster, and a conchoidal fracture. Colors are diverse and commonly influence their use in decorative objects and jewelry.

In petrographic study, cryptocrystalline textures are identified by the absence of visible crystal outlines under ordinary magnification and by microstructural features revealed with higher magnification. The term helps distinguish materials whose constituent crystals are too small to be seen in hand specimens from those with larger, well-formed crystals.