mineral
Chalcedony breccia (possibly agate with drusy quartz)
Microcrystalline quartz, SiO₂, with crystalline quartz and dark matrix/inclusions
AI-generated identification · may be incorrectHardness about 6.5–7 Mohs; translucent gray, cream, and brown zones with waxy to vitreous luster and sparkling drusy surfaces. Quartz has trigonal crystal symmetry, lacks cleavage, and has conchoidal fracture; the irregular shape appears naturally weathered or broken.
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Physical properties
Hardness about 6.5–7 Mohs; translucent gray, cream, and brown zones with waxy to vitreous luster and sparkling drusy surfaces. Quartz has trigonal crystal symmetry, lacks cleavage, and has conchoidal fracture; the irregular shape appears naturally weathered or broken.
Formation & geological history
Silica-rich groundwater deposited chalcedony and quartz in cavities or fractures, later incorporating angular dark fragments to form breccia-like banding. Such material commonly forms in volcanic rocks during the Cenozoic, though the specimen’s exact age and locality cannot be determined visually.
Uses & applications
Used as a lapidary stone, cabochon, carving material, and collector specimen; attractive sections may be polished for display. Common rough pieces generally have modest commercial value.
Geological facts
The glittering areas are likely drusy quartz—tiny quartz crystals coating a surface—while the opaque dark portions may be volcanic or iron/manganese-rich matrix.
Field identification & locations
Identify by testing for quartz hardness, conchoidal fracture, and acid resistance; it should scratch glass but not be scratched by a steel knife. Common sources include Brazil, Uruguay, Mexico, Madagascar, and the western United States.
