igneous
Basalt
Mafic Igneous Rock (composed primarily of pyroxene and calcium-rich plagioclase)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black (often weathered to light grey); Luster: Dull/earthy; Texture: Fine-grained (aphanitic) sometimes with visible porphyritic phenocrysts; Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black (often weathered to light grey)
- Luster
- Dull/earthy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black (often weathered to light grey); Luster: Dull/earthy; Texture: Fine-grained (aphanitic) sometimes with visible porphyritic phenocrysts; 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 specimen appears to be an extrusive igneous rock that has undergone significant surface weathering and smoothing, likely by water or glacial transport.
Uses & applications
Used extensively in construction as crushed stone for road base, railroad ballast, and concrete aggregate. Highly durable for paving stones and some sculptural uses.
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 found in abundance on the Moon and Mars. The white spots on this specific specimen are likely a combination of lichen growth and weathered mineral phenocrysts.
Field identification & locations
Identify in the field by its high density, dark interior color (when cracked), and fine-grained texture. It is commonly found in volcanic regions, oceanic islands, and mid-ocean ridges. Collectors look for amygdaloidal basalt which contains gas bubbles filled with secondary minerals like zeolites or quartz.
