igneous

Granite Cobble

Granite (Phaneritic Quartz-Feldspar specimen)

Hardness: 6-7 (Mohs scale); Color: Mottled brown, tan, and black; Lustre: Dull to vitreous; Crystal structure: Coarse-grained (phaneritic); Cleavage: Indistinct; Specific gravity: approx. 2.6 – 2.7.

Hardness: 6-7 (Mohs scale)
Color: Mottled brown, tan, and black

Identified March 22, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the slow cooling of silica-rich magma deep underground (plutonic). This specific specimen shows a rounded shape (cobble), indicating it was later eroded and transported by water or glacial activity.

Uses & applications

Used extensively in construction for countertops, floor tiles, and gravel. Polished pebbles are used in landscaping and aquarium decor.

Geological facts

Granite makes up the majority of the Earth's continental crust. It is primarily composed of quartz, fieldspar, and micas (biotite/muscovite). This specific specimen's brownish hue is likely due to iron oxide staining from weathering.

Field identification & locations

Identify by its interlocking 'salt and pepper' crystalline texture and extreme hardness (cannot be scratched by a steel knife). Commonly found in riverbeds, glacial moraines, and mountain ranges.

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Blue Apatite

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