mineral
Botryoidal Chalcedony / Opal
Silicon Dioxide (SiO2 · nH2O)
Hardness 5.5-7 on Mohs scale; pale blue, white, to iridescent sheen; waxy to vitreous luster; cryptocrystalline or amorphous structure; no cleavage; specific gravity ~2.1-2.6.
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Physical properties
Hardness 5.5-7 on Mohs scale; pale blue, white, to iridescent sheen; waxy to vitreous luster; cryptocrystalline or amorphous structure; no cleavage; specific gravity ~2.1-2.6.
Formation & geological history
Formed at low temperatures from silica-rich waters precipitating in cavities, fractures, and volcanic rocks, often forming microcrystalline botryoidal (grape-like) clusters or opalized crusts.
Uses & applications
Popular among lapidaries and mineral collectors for jewelry, polished cabochons, decorative carvings, and display specimens.
Geological facts
Botryoidal texture takes its name from the Ancient Greek word for 'bunch of grapes'. When displaying adularescence or play-of-color, such specimens transition into precious opal territory.
Field identification & locations
Identified in the field by its rounded, bubble-like surface texture, hardness (resists scratching with copper or steel), and translucent body color. Commonly found in silica-rich volcanic environments in the western US, Mexico, Brazil, and Madagascar.
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