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  2. Porphyry (geology) - Wikipedia

    en.wikipedia.org/wiki/Porphyry_(geology)

    [1] [2] The term porphyry is from the Ancient Greek πορφύρα (porphyra), meaning "purple". Purple was the colour of royalty, and the Roman "imperial porphyry" was a deep purple igneous rock with large crystals of plagioclase. Some authors claimed the rock was the hardest known in antiquity. [3]