A mineral or rock whose internal structure or composition is hidden or difficult to determine, either because it's amorphous or because its crystalline structure is microscopic.
From Greek 'kryptos' (hidden) + '-lite' (stone), a mineralogical term from the 1800s describing rocks whose true nature couldn't be determined by the naked eye.
Obsidian glass looks perfectly uniform to the naked eye, but it's actually a cryptolite—hidden inside are tiny crystalline structures that only powerful microscopes can reveal, showing that what seems smooth might be secretly complex.
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