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1 Chalcedonic (see chalcedony) variety of cryptocrystalline silica, SiO2, that occurs as nodules or irregular masses in a sedimentary environment, often in association with black shales and spilites.

2 A fine-grained rock consisting of beds of cryptocrystalline silica, usually of biogenic, volcanogenic, or diagenetic (see diagenesis) origin.

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