igneous
Porphyritic Granite (Glacial Erratic)
Porphyritic Biotite Granite
Hardness: 6-7 Mohs; Color: Pinkish-red matrix with large black (hornblende/biotite) and white/grey (quartz) crystals; Luster: Dull to vitreous; Structure: Phaneritic/Porphyritic (visible large crystals); Specific Gravity: 2.6-2.7
- Hardness
- 6-7 Mohs
- Luster
- Dull to vitreous
Identified More igneous →
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Physical properties
Hardness: 6-7 Mohs; Color: Pinkish-red matrix with large black (hornblende/biotite) and white/grey (quartz) crystals; Luster: Dull to vitreous; Structure: Phaneritic/Porphyritic (visible large crystals); Specific Gravity: 2.6-2.7
Formation & geological history
Formed from the slow cooling of silica-rich magma deep underground, often dating back to the Proterozoic or Archean eons. This specific specimen is likely a glacial erratic, transported far from its source by ice sheets during the last Ice Age.
Uses & applications
Used in high-end construction, kitchen countertops, monument carving, and as decorative landscaping boulders.
Geological facts
Granite is the primary component of continental crust. The pink color comes from potassium feldspar (orthoclase). This boulder is known as an 'erratic' because its composition differs from the local bedrock where it was deposited.
Field identification & locations
Identify by the 'salt and pepper' look with distinct pink feldspar crystals. Often found in glacial till or as solitary boulders in fields. Collectors look for unique phenocrysts (large crystals) and rounded weather patterns.
