igneous

Granite Cobble

Phaneritic Plutonic Felsic Rock

Hardness: 6-7 Mohs; Color: Pinkish-orange (K-feldspar), grey/translucent (quartz), and black/dark brown (biotite/amphibole); Luster: Dull/Waxy (weathered) to Vitreous (fresh surfaces); Grain: Medium to coarse phaneritic crystals; Specific Gravity: 2.63 - 2.75.

Hardness: 6-7 Mohs
Luster: Dull/Waxy (weathered) to Vitreous (fresh surfaces)

Identified March 15, 2026More igneous →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed through the slow cooling and crystallization of silica-rich magma deep within the Earth's crust (plutonism). This specimen has been rounded through fluvial or glacial erosion, as evidenced by its smooth, water-worn shape. Age can range from hundreds of millions to billions of years depending on the regional craton.

Uses & applications

Used widely in construction (crushed stone, aggregate, dimension stone), kitchen countertops when quarried in slabs, and as decorative landscaping stones. Smooth river-worn cobbles are often used in masonry and landscaping.

Geological facts

Granite makes up much of the continental crust. Because it is high in quartz and feldspar, it is exceptionally resistant to weathering compared to other rocks, which is why it often survives as rounded cobbles long after softer rocks have eroded away.

Field identification & locations

Identify in the field by its 'salt and pepper' appearance (mixed light and dark crystals) and its inability to be scratched by a steel knife. It is found globally, especially in mountain ranges, shield areas like Canada/Africa, and stream beds or glacial till. Collectors look for unique mineral inclusions or rare colors.

Other igneous specimens

See all

Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous