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|Name:||Named by Ernest Mallard in 1890 for a chalcedony-like material with unique properties from Lussat, near Pont-du-Chateau, Puy-de-Dome. Associated with opal and quartz.|
"A form of silica, similar in structure to chalcedony, but having a low specific gravity, G = 2.04, near that of opal; refractive index (for D) 1.446 like opal, but crystalline and optically positive." Mallard, 1890.