Describing a mineral or rock whose crystal structure is so fine that individual crystals cannot be seen without a microscope.
From 'crypto-' (hidden) + 'crystalline' (from Latin 'crystallum'). Mineralogists adopted this term in the 1800s to classify minerals with microscopic crystal structures.
Chalcedony and flint are cryptocrystalline—they look like smooth glass but are actually made of countless tiny quartz crystals packed so tightly you'd need a scanning electron microscope to see them individually!
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