metamorphic

Rounded metamorphic cobble, likely gneiss or amphibolite

Banding-free biotite–quartz–feldspar gneiss or amphibolite; exact identification requires testing

AI-generated identification · may be incorrect

Dark gray-brown, fine- to medium-grained, granular texture with scattered pale mineral grains and possible mica; likely Mohs 6–7, vitreous to dull luster, nonporous, with no obvious cleavage at specimen scale. The elongated, smoothly rounded form indicates water or glacial abrasion.

Identified September 4, 2026More metamorphic →

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At a glance

Common name: Rounded metamorphic cobble, likely gneiss or amphibolite
Scientific name: Banding-free biotite–quartz–feldspar gneiss or amphibolite; exact identification requires testing
Category: metamorphic
Identified: September 4, 2026
Method: Standard

AI Estimated Value

Approximately $1–$15 as an ordinary rounded cobble; up to about $20–$50 if unusually attractive, labeled, or from a notable locality.

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 by deep burial and heat/pressure during regional metamorphism, probably from older igneous or sedimentary protoliths; its precise geological age cannot be determined from the photograph. Later erosion shaped it into a stream or glacial cobble.

Uses & applications

Primarily a field specimen or decorative stone; dense metamorphic rock may be used locally as aggregate, riprap, landscaping stone, or building material, but this piece has little gem value.

Geological facts

The pale flecks may be quartz and feldspar, while dark grains may be biotite, hornblende, or magnetite. A streak, hardness, acid, magnetism, and thin-section tests would distinguish gneiss from amphibolite or other dark metamorphic rocks.

Field identification & locations

Common in metamorphic terrains and downstream river gravels worldwide, especially regions containing Precambrian crystalline bedrock. Look for mineral banding, scratch resistance against glass, and possible magnetism; visual identification alone is uncertain.