metamorphic

Garnet-bearing amphibolite or mafic gneiss

Amphibolite dominated by hornblende, plagioclase, biotite, and almandine–pyrope garnet

AI-generated identification · may be incorrect

Identified September 13, 2026More metamorphic →

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Physical properties

Dark green-black, coarse-grained, with reddish-brown garnet crystals and pale plagioclase; generally vitreous to dull luster, granular texture, Mohs hardness about 5.5–7.5, and no consistent cleavage in the whole rock. Garnet is commonly dodecahedral and lacks cleavage, while amphibole shows two cleavages near 60°/120°.

Formation & geological history

Formed when basaltic or gabbroic protoliths underwent medium- to high-grade regional metamorphism, commonly during mountain building; individual specimens may range from Precambrian to younger geological ages. The image may also represent garnet-bearing amphibolite rather than true gneiss because foliation is weak.

Uses & applications

Used locally as crushed stone, riprap, and decorative stone; attractive garnet-bearing pieces are collected as mineral specimens. Garnet itself is also used as an abrasive and gemstone.

Geological facts

Garnet’s reddish crystals contrast strongly with dark amphibole, making this a classic metamorphic assemblage. Common locations include metamorphic belts in North America, Scandinavia, India, and southern Africa.

Field identification & locations

Identify by the dark amphibole-rich matrix, scattered hard reddish garnets, coarse texture, and resistance to acid; a streak, hardness, and thin-section test can confirm minerals. Avoid assuming every red grain is garnet—weathered biotite or feldspar may look similar.