igneous

Granite Cobble

Felsic intrusive igneous rock (composition: SiO₂ with KAlSi₃O₈, NaAlSi₃O₈, and CaAl₂Si₂O₈)

Hardness: 6-7 on Mohs scale. Color: Mottled grey, white, and pinkish-tan with dark specks. Luster: Dull (weathered) to vitreous (on fresh breaks). Structure: Phaneritic (coarse-grained) crystalline. Cleavage: Indistinct in hand specimen.

Hardness: 6-7 on Mohs scale
Color: Mottled grey, white, and pinkish-tan with dark specks
Luster: Dull (weathered) to vitreous (on fresh breaks)

Identified March 22, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the slow cooling of silica-rich magma deep within the Earth's crust (plutonic). This specific specimen shows a rounded shape indicative of fluvial or glacial transport during the Quaternary period, which has smoothed its edges.

Uses & applications

Used extensively in construction as dimension stone, architectural veneer, road base (crushed), and as high-end kitchen countertops. Lapidary use includes polishing for decorative heart-stones or paperweights.

Geological facts

Granite makes up the bulk of the Earth's continental crust. It is one of the hardest and most durable common stones, which is why it often survives as a 'cobble' long after softer rocks have eroded away into sand.

Field identification & locations

Identify by its 'salt and pepper' appearance and visible interlocking crystals of quartz, feldspar, and mica. Commonly found in glacial till, riverbeds, and mountainous regions. Collectors look for unique mineral inclusions or striking color patterns.

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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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Magnetite (Fe3O4)

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