A calcium-rich mineral or compound, sometimes used historically to refer to chalcedony or other silicate minerals found in limy regions.
From Greek 'calcis' (lime) + 'sedone' (chalcedony), though the etymology is debated among mineralogists. It may represent an archaic term for calcium silicates.
Calcydon sits at the intersection of limestone and precious stones—it's a term that shows how ancient mineralogists were trying to classify rocks that seemed to bridge the worlds of common and rare!
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