igneous
Basalt River Cobble
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 5-6 Mohs; Color: Dark grey, charcoal to brown (due to weathering); Luster: Dull/Earthy; Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8–3.0
- Hardness
- 5-6 Mohs
- Color
- Dark grey, charcoal to brown (due to weathering)
- Luster
- Dull/Earthy
Identified More igneous →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 5-6 Mohs; Color: Dark grey, charcoal to brown (due to weathering); Luster: Dull/Earthy; Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. This specific specimen has been rounded and smoothed by fluvial (river) erosion over many years.
Uses & applications
Used primarily in construction as aggregate, for road base, or in landscaping. Weathered basalt cobbles are also popular in decorative gardens and as massage stones.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also extremely common on the Moon and Mars.
Field identification & locations
Identify in the field by its heavy weight (density), dark interior, and lack of visible crystals. Commonly found in riverbeds, volcanic fields, and coastal areas.
