metamorphic

Polished dark metamorphic cobble

Likely amphibolite or hornblende-rich schist; exact identification requires thin-section or chemical testing

AI-generated identification · may be incorrect

Dark gray-black, fine- to medium-grained, with subtle bronzy-brown streaks and a satiny luster from polishing; likely Mohs 5–6, with weak foliation and no obvious cleavage visible on the rounded surface. Composition is probably hornblende, plagioclase, biotite, and minor iron oxides.

Identified September 11, 2026More metamorphic →

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At a glance

Common name: Polished dark metamorphic cobble
Scientific name: Likely amphibolite or hornblende-rich schist; exact identification requires thin-section or chemical testing
Category: metamorphic
Identified: September 11, 2026
Method: Standard

AI Estimated Value

Usually $5–$25 as a polished common cobble; exceptional provenance, pattern, or craftsmanship may bring $30–$75.

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

Formed when basaltic or gabbroic rock underwent medium- to high-grade regional metamorphism, commonly during Precambrian or Paleozoic mountain-building; the specimen was later rounded by stream or glacial transport and polished.

Uses & applications

Used as decorative stone, landscaping material, polished display specimens, and occasionally aggregate; generally not a gemstone.

Geological facts

Amphibolite can resemble dark basalt, but aligned amphibole crystals or faint foliation distinguish many specimens. Brown-gold zones may reflect weathered biotite or iron-oxide staining.

Field identification & locations

Common in metamorphic belts and glacial or river gravels worldwide. Look for elongated hornblende grains, faint planar alignment, dense weight, and a scratch resistance near that of steel; acid testing is unhelpful.