Igneous
Greenstone / Diabase Porphyry
Porphyritic Diabase (Micro-gabbro)
Hardness: 6-7 (Mohs). Color: Dark green to grey-green matrix with darker green phenocrysts. Luster: Dull to waxy when wet/polished. Structure: Fine-grained matrix with larger, visible crystal clusters (porphyritic texture). Specific gravity: 2.8 - 3.2.
- Hardness
- 6-7 (Mohs)
- Luster
- Dull to waxy when wet/polished
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Physical properties
Hardness: 6-7 (Mohs). Color: Dark green to grey-green matrix with darker green phenocrysts. Luster: Dull to waxy when wet/polished. Structure: Fine-grained matrix with larger, visible crystal clusters (porphyritic texture). Specific gravity: 2.8 - 3.2.
Formation & geological history
Formed from the slow cooling of mafic magma (basaltic composition) within the Earth's crust, typically in dikes or sills. The distinct green color is often the result of low-grade metamorphism (greenschist facies) where minerals like chlorite, epidote, or actinolite replace primary minerals.
Uses & applications
Used primarily in construction as crushed stone or road gravel. Polished specimens are used in lapidary work, decorative landscaping, and collecting. Historically used for making stone tools due to its toughness.
Geological facts
Greenstone is a general field term for any dark, greenish-colored igneous rock. The darker spots in this specimen are likely concentrations of augite or hornblende crystals. Ancient cultures often favored greenstones for ceremonial axes.
Field identification & locations
Identify in the field by its characteristic 'dull' green hue and high density. It lacks the layering of metamorphic slate and is harder than most sedimentary rocks. Commonly found in areas of ancient volcanic activity such as the Lake Superior region or the Appalachian Mountains.
