igneous
Basalt with Iron Oxidation
Amygdaloidal Basalt (silicate rock primarily plagioclase, pyroxene, and olivine)
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal with reddish-brown oxidation streaks; Luster: Dull to sub-vitreous when wet; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to charcoal with reddish-brown oxidation streaks
- Luster
- Dull to sub-vitreous when wet
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal with reddish-brown oxidation streaks; Luster: Dull to sub-vitreous when wet; 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. The reddish streaks indicate secondary iron oxidation (rusting) occurring along micro-fractures during weathering.
Uses & applications
Used in construction as aggregate, for paving, in the manufacture of rock wool insulation, and occasionally as decorative river stones in landscaping.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up the majority of the ocean floor. It is also found extensively on the Moon and Mars.
Field identification & locations
Identify in the field by its heavy weight, dark color, and lack of visible crystals. This specimen is water-worn and polished, likely found in a riverbed or coastal area. Collectors should look for signs of vesicles (gas bubbles) or trapped minerals.
