igneous
Granite
Granite (Phaneritic Igneous Rock)
Hardness: 6-7 Mohs; Color: Mottled pink, white, and black; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Cleavage: Distinct in feldspar, none in quartz; Specific gravity: 2.63-2.75
- Hardness
- 6-7 Mohs
- Color
- Mottled pink, white, and black
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Mottled pink, white, and black; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Cleavage: Distinct in feldspar, none in quartz; Specific gravity: 2.63-2.75
Formation & geological history
Formed from the slow crystallization of magma below Earth's surface (intrusive). While the surface geology at the found location (Iowa/Illinois border) is primarily Paleozoic sedimentary rock, this specimen is likely a glacial erratic—transported by ice sheets from the Canadian Shield during the Pleistocene epoch.
Uses & applications
Used in construction, countertops, monuments, and as crushed stone for road base.
Geological facts
Granite is the most common rock in the continental crust. The pink color is caused by potassium feldspar, while the clear/white is quartz and the black is usually biotite mica or hornblende.
Field identification & locations
Identify by its interlocking 'salt and pepper' crystalline texture and high hardness (it will scratch glass). In this specific region, keep an eye out for larger boulders of similar composition appearing out of place in fields or riverbeds, which are signs of prehistoric glacial paths.
