mineral

Tumbled black stone, likely black agate or onyx

Microcrystalline quartz (SiO₂), probably chalcedony/black agate; exact identification requires testing

AI-generated identification · may be incorrect

Glossy, smoothly tumbled, opaque black with a thin pale gray-white curved band. Quartz hardness is about 6.5–7 Mohs, vitreous to waxy luster, cryptocrystalline structure, no visible cleavage, and specific gravity near 2.6.

Identified September 17, 2026More mineral →

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At a glance

Common name: Tumbled black stone, likely black agate or onyx
Scientific name: Microcrystalline quartz (SiO₂), probably chalcedony/black agate; exact identification requires testing
Category: mineral
Identified: September 17, 2026
Method: Standard

AI Estimated Value

About $2–$15 for a small common tumbled stone; $20–$75 for unusually patterned, natural, or larger specimens.

Not an official valuation

AI estimate for general interest only — not an appraisal. Actual value depends on condition, provenance, and market demand. Verify with a professional before buying, selling, or insuring.

This identification was generated by AI and may contain errors. Verify important details before relying on them — and never use an AI result to judge whether a specimen is safe to handle. Read the full disclaimer

Physical properties

Formation & geological history

Agate forms when silica-rich groundwater fills cavities in volcanic rock and deposits layered chalcedony; black coloration may result from iron, manganese, organic matter, or dyeing. Deposits range from Paleozoic to Cenozoic in age.

Uses & applications

Used for cabochons, beads, carvings, worry stones, and decorative objects; larger agate slabs are used ornamentally. Common tumbled pieces have modest value.

Geological facts

The pale line resembles agate banding, but black onyx, dyed chalcedony, or slag-like material can look similar. A glassy luster, cool feel, and lack of bubbles support quartz rather than glass.

Field identification & locations

Commonly found in Brazil, Uruguay, India, Madagascar, Mexico, and the United States. Check for banding under magnification, test hardness against glass, and inspect for dye concentration in cracks or pores.