A historic term for a transparent or translucent mineral, historically believed to be a type of selenite or a glass-like stone.
From Greek 'phengites' (shining) derived from 'phengos' (light); medieval and ancient scholars used this term for luminous minerals they believed had special optical properties.
Ancient Romans supposedly used fengite (probably a type of selenite crystal) as 'Lapis Specularis' for window panes because it was transparent—imagine having crystal windows in your Roman villa!
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