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
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Physical properties
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.
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).
