igneous
Granite Pebble
Phaneritic Felsic Igneous Rock (Granite)
Hardness: 6-7 Mohs; Color: Mottled pink, black, and gray; Luster: Dull to vitreous; Crystal structure: Phaneritic (large visible crystal grains of quartz, feldspar, and mica); Cleavage: Dependent on mineral (poor for quartz, two at 90 deg for feldspar); Specific gravity: 2.63-2.75
- Hardness
- 6-7 Mohs
- Color
- Mottled pink, black, and gray
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Mottled pink, black, and gray; Luster: Dull to vitreous; Crystal structure: Phaneritic (large visible crystal grains of quartz, feldspar, and mica); Cleavage: Dependent on mineral (poor for quartz, two at 90 deg for feldspar); Specific gravity: 2.63-2.75
Formation & geological history
Formed through the slow crystallization of magma below Earth's surface (intrusive). This specimen specifically has been rounded by fluvial (water) or glacial erosion into a pebble.
Uses & applications
Commonly used in large scale for construction, countertops, and monuments; as a small pebble, it is used for landscaping, decorative aquariums, and teaching petrology.
Geological facts
Granite makes up the majority of Earth's continental crust. The pink color in this specimen is likely due to potassium feldspar (orthoclase), while the dark spots are biotite mica or hornblende.
Field identification & locations
Identify by looking for a 'salt and pepper' or mottled appearance with visible interlocking crystals. It is extremely common in areas with exposed batholiths or glacial till. Pebbles like this are easily found in riverbeds or gravel pits.
