igneous
Gray Granite / Rhyolite
Felsic Igneous Rock (Granitoid)
Hardness: 6-7 on Mohs scale; Color: light gray to salt-and-pepper; Luster: dull to earthy (weathered surface); Crystal structure: phaneritic to aphanitic depending on cooling rate; Cleavage: none; Specific gravity: 2.6-2.7
- Hardness
- 6-7 on Mohs scale
- Color
- light gray to salt-and-pepper
- Luster
- dull to earthy (weathered surface)
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Physical properties
Hardness: 6-7 on Mohs scale; Color: light gray to salt-and-pepper; Luster: dull to earthy (weathered surface); Crystal structure: phaneritic to aphanitic depending on cooling rate; Cleavage: none; Specific gravity: 2.6-2.7
Formation & geological history
Formed through the slow or rapid cooling of silica-rich magma/lava. If coarse-grained, it cooled underground (Granite); if fine-grained, it erupted (Rhyolite). These rocks can date back billions of years or be as young as millions.
Uses & applications
Used extensively in construction as road base, concrete aggregate, and dimension stone for countertops and monuments. Smaller pebbles are often used in landscaping.
Geological facts
Granite is the primary component of Earth's continental crust. It is highly resistant to weathering compared to other rock types, which is why it often remains as pebbles while other rocks erode away.
Field identification & locations
Look for a granular texture with light-colored minerals like quartz and feldspar. To identify in the field, check for hardness (cannot be scratched by a steel nail) and the presence of small reflective mica flakes. It is found globally in mountainous regions and riverbeds.
