metamorphic

Likely quartz-rich rock, possibly milky quartz or quartzite

Quartz (SiO₂); if granular and recrystallized, quartzite

AI-generated identification · may be incorrect

Pale gray-brown, translucent to opaque, with a dull-to-glassy luster and irregular fracture; visible grain/texture is limited. Quartz hardness is Mohs 7, no cleavage, and specific gravity about 2.65; quartzite is similarly hard and compact.

Identified September 14, 2026More metamorphic →

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Physical properties

Formation & geological history

Could be a fragment of vein quartz or metamorphosed sandstone (quartzite), formed by silica-rich fluids or recrystallization during metamorphism. The specimen’s geological age cannot be determined visually; quartz-rich rocks are common around the San Francisco Bay region.

Uses & applications

Quartz is used in glass, electronics, abrasives, and engineered stone; quartzite is used as aggregate, building stone, and decorative slabs. This small, ordinary fragment has little commercial value.

Geological facts

The blocky shape and pale translucent areas favor quartz, but the brown surface may be iron staining or weathering. A steel nail should not scratch it; quartz will scratch glass.

Field identification & locations

Common regional possibilities include vein quartz, quartzite, and silica-rich metamorphic rocks associated with Franciscan Complex terrains. Confirm with a hardness test, streak (white), and inspection for granular texture versus glassy fracture.