metamorphic

Weathered mafic metamorphic rock, likely amphibolite or hornblende gneiss

Amphibolite; primarily hornblende amphibole and plagioclase feldspar, with possible biotite, quartz, and iron oxides

AI-generated identification · may be incorrect

Dark gray to brown, coarse crystalline, with a speckled/granular texture and dull to slightly vitreous luster; hardness about 5–6, typically massive to weakly foliated, with no obvious cleavage at specimen scale. Brown surfaces likely represent oxidation and weathering.

Identified September 14, 2026More metamorphic →

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At a glance

Common name: Weathered mafic metamorphic rock, likely amphibolite or hornblende gneiss
Scientific name: Amphibolite; primarily hornblende amphibole and plagioclase feldspar, with possible biotite, quartz, and iron oxides
Category: metamorphic
Identified: September 14, 2026
Method: Standard

AI Estimated Value

About $5–$25 for a common hand specimen; $25–$75 for an attractive polished or unusually patterned piece.

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 basalt or gabbro underwent medium- to high-grade regional metamorphism, commonly during Precambrian to Paleozoic mountain-building events. The rusty patches developed through later weathering of iron-bearing minerals.

Uses & applications

Used as crushed aggregate, dimension stone, riprap, and occasionally architectural stone; attractive polished pieces may be collected but are generally not a jewelry gemstone.

Geological facts

Amphibolite is identified by abundant dark hornblende surrounded by lighter plagioclase; it commonly preserves the chemical heritage of ancient volcanic rocks despite metamorphic recrystallization.

Field identification & locations

Common in metamorphic belts and exposed basement terrains worldwide, including Scandinavia, Canada, the northeastern United States, and the Appalachians. A definitive identification requires thin-section microscopy or mineral testing because the image could also show weathered gneiss or altered basalt.