igneous

Granite (Specimen: Logan Rock)

Phaneritic plutonic igneous rock (mainly Quartz, Feldspar, and Mica)

Hardness: 6-7 (Mohs scale); Color: Gray, pink, or white mottled with black; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained, visible crystals); Cleavage: None in the rock itself, though constituent minerals have it; Specific gravity: 2.63 - 2.75.

Hardness: 6-7 (Mohs scale)
Color: Gray, pink, or white mottled with black
Luster: Vitreous to dull

Identified March 23, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the slow crystallization of silica-rich magma below the Earth's surface. This specific specimen in Cornwall, England (Treryn Dinas), is part of the Cornubian Batholith, formed approximately 280-290 million years ago during the Variscan orogeny.

Uses & applications

Extensively used in construction for countertops, monuments, paving stones, and as aggregate in road building. Large boulders like the one shown are significant for tourism and geological study.

Geological facts

This specific boulder is the famous 'Logan Rock' in Treen, Cornwall. It is a 'rocking stone'—a 65-ton boulder so perfectly balanced that it could historically be moved with a gentle touch. In 1824, members of the British Royal Navy tipped it over, and it had to be re-installed at great expense.

Field identification & locations

Identify by its coarse, interlocking crystals of quartz (clear/gray), feldspar (white/pink), and mica (shiny black/silver). Found in continental crust worldwide. Collectors look for unique inclusions or aesthetic weathering (tors).

Other igneous specimens

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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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral