igneous
Polished reddish-brown granite or granitic gneiss slab
Granite, chiefly quartz, feldspar, and mica; possibly foliated as gneiss
AI-generated identification · may be incorrectCoarse-grained, mottled gray, pink, and brown with black biotite/hornblende; generally vitreous to dull, Mohs hardness about 6–7, no obvious cleavage as a rock, and specific gravity about 2.6–2.8.
Identified More igneous →
Explore Polished reddish-brown granite or granitic gneiss slab in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Coarse-grained, mottled gray, pink, and brown with black biotite/hornblende; generally vitreous to dull, Mohs hardness about 6–7, no obvious cleavage as a rock, and specific gravity about 2.6–2.8.
Formation & geological history
Granite crystallizes slowly from silica-rich magma deep in the crust; gneiss forms when granite is intensely metamorphosed. Such rocks are commonly Precambrian to Mesozoic, depending on locality.
Uses & applications
Widely used as dimension stone, countertops, monuments, flooring, aggregate, and architectural slabs; attractive polished pieces have modest decorative or lapidary value.
Geological facts
The large visible crystals indicate slow underground cooling, while dark streaks or banding may reflect metamorphic foliation. Exact identification requires a fresh surface and mineral-grain examination.
Field identification & locations
Common in continental mountain belts and exposed Precambrian shields worldwide. Identify by interlocking quartz-feldspar grains, pink feldspar, gray quartz, and black mica; banding favors gneiss over granite.
