igneous

Granite Cobble

Granite (Phaneritic Quartz-Feldspar-Mica intrusive rock)

Hardness: 6-7 (Mohs). Color: Salt-and-pepper (white/grey with black specks). Luster: Dull (weathered) to vitreous (on fresh surface). Structure: Medium-grained phaneritic. Specific Gravity: 2.63 - 2.75.

Hardness: 6-7 (Mohs)
Color: Salt-and-pepper (white/grey with black specks)
Luster: Dull (weathered) to vitreous (on fresh surface)

Identified March 21, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the slow cooling of silica-rich magma deep underground (plutonic). This specimen has been rounded by fluvial or coastal erosion, likely over thousands of years.

Uses & applications

Construction material, dimension stone, landscaping, and aggregate. In this small, rounded form, it is used for decorative gardens or as worry stones.

Geological facts

Granite makes up a large portion of the Earth's continental crust. Because of its hardness and resistance to erosion, it often persists as rounded cobbles long after softer rocks have broken down into sand.

Field identification & locations

Identify in the field by its 'salt and pepper' appearance and visible interlocking crystals of quartz, feldspar, and biotite. Found globally in riverbeds, glacial tills, and beaches.

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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

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