igneous

Granite Cobble

Phaneritic Felsic Igneous Rock (Granite)

Hardness: 6-7 on Mohs scale; Color: Mottled grey, pink, and black; Luster: Dull (weathered/water-worn); Crystal Structure: Prismatic/Granular; Cleavage: None (fractures unevenly); Specific Gravity: 2.6-2.7

Hardness: 6-7 on Mohs scale
Color: Mottled grey, pink, and black
Luster: Dull (weathered/water-worn)

Identified April 11, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the slow crystallization of magma below Earth's surface. This specific specimen has been smoothed and rounded into a 'cobble' or 'river rock' shape through fluvial (water) or glacial erosion over thousands of years.

Uses & applications

Used primarily in landscaping, decorative masonry, and as high-quality aggregate in construction. Granite is also the source material for many kitchen countertops and architectural monuments.

Geological facts

Granite makes up a large part of the Earth's continental crust. Because it is highly resistant to weathering, these cobbles can survive long journeys in rivers or glacial till, retaining their hardness even when rounded.

Field identification & locations

Identify in the field by its 'salt and pepper' appearance and visible interlocking crystals of quartz, feldspar, and mica. This rounded shape indicates it was found in a riverbed, beach, or glacial deposit.

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Fluorapatite (Ca5(PO4)3F)

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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

Magnetite

Iron(II,III) oxide (Fe3O4)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

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Magnetite

Magnetite (Fe3O4)

mineral