Transparent or opaque? These are crystals of the antimony ore stibnite (antimony sulphide). While they are grey and opaque to our eyes, they are actually transparent to infra-red light.

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Photo credit: © World History Archive / Alamy / Afripics
License: Licensed
Model Released: No

Keywords: ., antimony, archival, archive, crystals, eyes, geology, grey, historical, history, infra-red, light., opaque, opaque?, ore, rock, stibnite, transparent