igneous
Basalt with Epidote (Greenstone)
Amygdaloidal Basalt (Ca2(Al, Fe)3(SiO4)3(OH) inclusions)
Hardness: 6-7 (Mohs). Color: Dark greyish-green to hunter green with small vesicles or patches. Luster: Dull/earthy (weathered) to vitreous (minerals). Structure: Aphanitic with porphyritic amygdules. Specific gravity: 2.8–3.0.
- Hardness
- 6-7 (Mohs)
- Luster
- Dull/earthy (weathered) to vitreous (minerals)
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Physical properties
Hardness: 6-7 (Mohs). Color: Dark greyish-green to hunter green with small vesicles or patches. Luster: Dull/earthy (weathered) to vitreous (minerals). Structure: Aphanitic with porphyritic amygdules. Specific gravity: 2.8–3.0.
Formation & geological history
Formed from rapid cooling of magnesium-rich and iron-rich lava at the surface. The green tint is often due to low-grade metamorphism (greenschist facies) where minerals like chlorite and epidote replace original mafic minerals.
Uses & applications
Commonly used in construction as crushed stone or aggregate. High-quality greenstone specimens are used in lapidary work for cabochons, polished river stones, and decorative landscaping.
Geological facts
Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor. When weathered by water, it often gains a characteristic rounded shape and a smooth, almost 'waxy' feel if rich in fine-grained silica.
Field identification & locations
Identify by its heavy weight relative to its size and its dark, greenish-black hue. Found often on beaches near volcanic coastlines (Pacific Northwest, Great Lakes). Collectors should look for 'amygdules' (filled gas bubbles) that may contain quartz or agate.
