igneous
Basalt (Greenstone)
Basalt with chlorite/epidote (often referred to as Greenstone)
Hardness: 6-7 (Mohs scale), Color: Dark dull green to grayish-green, Luster: Dull to waxy when wet, Crystal structure: Fine-grained/aphanitic, often showing small pits or vesicles, Specific gravity: 2.8–3.0
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Dark dull green to grayish-green, Luster: Dull to waxy when wet, Crystal structure: Fine-grained/aphanitic, often showing small pits or vesicles, Specific gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The green color is a result of low-grade metamorphism or hydrothermal alteration where minerals like chlorite, epidote, or actinolite replace original volcanic minerals.
Uses & applications
Used primarily as crushed stone for construction, road base, and railroad ballast. Exceptionally smooth pieces are valued by pebble collectors and for lapidary use.
Geological facts
Basalt is the most common rock in the Earth's crust and makes up most of the ocean floor. When basalt undergoes mild heat and pressure, it turns into 'greenstone,' a stone that was historically used for making axes by ancient cultures.
Field identification & locations
Identify by its heavy weight (relative to size), dark green hue, and smooth, water-worn texture often found on beaches or rivers. It will not be scratched by a steel knife easily.
