Igneous
Basalt (Greenstone)
Amygdaloidal Basalt / Chloritized Basalt
Hardness: 5-6 (Mohs); Color: Dark greenish-gray to dull olive green; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8-3.0.
- Hardness
- 5-6 (Mohs)
- Color
- Dark greenish-gray to dull olive green
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark greenish-gray to dull olive green; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8-3.0.
Formation & geological history
Formed through the rapid cooling of basaltic lava at the Earth's surface. The green tint is caused by low-grade metamorphism or hydrothermal alteration where primary minerals like pyroxene turn into chlorite, epidote, or actinolite. Common in Precambrian and Paleozoic volcanic belts.
Uses & applications
Used primarily as crushed stone for road base, concrete aggregate, and railroad ballast. Occasionally used as garden 'landscape rock' or for minor ornamental lapidary work if it contains unique inclusions.
Geological facts
Basalt is the most common rock type on Earth's crust and makes up the majority of the ocean floor. When basalt turns green, it is often referred to by geologists as 'greenstone,' indicating it has undergone low-pressure metamorphism typical of ancient seafloors.
Field identification & locations
Identify in the field by its heavy weight (density), dark color, and fine-grained texture that lacks visible large crystals. It often appears 'veiny' or slightly mottled. Found frequently in glacial till or riverbeds in volcanic regions.
