mineral
Polished dark agate or jasper specimen
Cryptocrystalline quartz (chalcedony), SiO₂; exact identification is uncertain from the image
AI-generated identification · may be incorrectDark brown-to-black, irregularly patterned, waxy to subvitreous luster; hardness about 6.5–7 Mohs, no obvious cleavage, microcrystalline quartz structure, and specific gravity near 2.6. The surface appears polished or tumbled.
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Physical properties
Dark brown-to-black, irregularly patterned, waxy to subvitreous luster; hardness about 6.5–7 Mohs, no obvious cleavage, microcrystalline quartz structure, and specific gravity near 2.6. The surface appears polished or tumbled.
Formation & geological history
Likely formed when silica-rich fluids filled cavities in volcanic rock, later developing chalcedony and possibly iron/manganese staining; commonly Cenozoic to older, depending on locality.
Uses & applications
Used for cabochons, beads, carvings, ornamental objects, and lapidary specimens; not generally important as a construction stone in this form.
Geological facts
Agate and jasper can grade into one another: banded, translucent material is typically called agate, while opaque dark material is often marketed as jasper.
Field identification & locations
Identify by its quartz-like hardness, nonmetallic waxy polish, lack of cleavage, and inability to scratch with a steel blade; field certainty requires a close, unblurred view and ideally translucency or streak testing. Common worldwide, especially in volcanic terrains.
