metamorphic

Hornblende-biotite schist (probable)

Amphibole–biotite schist; chiefly hornblende, biotite, plagioclase, and quartz

AI-generated identification · may be incorrect

Dark green-black, coarse and strongly foliated, with shiny mica/amphibole faces and pale granular patches. Typical hardness is about 5–6; luster is vitreous to pearly, with schistose texture and weak-to-good cleavage along foliation.…

Identified September 13, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when a basaltic or muddy protolith was metamorphosed under medium-grade greenschist-to-amphibolite-facies conditions, commonly during regional mountain-building. Its age cannot be determined from the image; rocks of comparable composition occur in Southern California’s older metamorphic basement and fault-zone terranes.

Uses & applications

Primarily a collector or teaching specimen; massive varieties may be used as crushed aggregate or decorative stone, but foliated schist is generally unsuitable for structural construction.

Geological facts

The bright reflective flakes are likely biotite, while green-black prismatic grains may be hornblende. The nearby reddish slabs could be unrelated weathered rock or iron-stained metamorphic material.

Field identification & locations

Look for planar foliation, mica sparkle, and dark amphibole-rich layers; distinguish it from basalt by its visible aligned minerals and split surfaces. Around 34.1°N, 117.6°W, metamorphic rocks are geologically plausible, though a field test or thin section is needed to separate hornblende schist from chlorite schist or altered basalt.