igneous
Greenstone Porphyry
Altered Porphyritic Andesite/Basalt
Hardness: 6-7 (Mohs scale); Color: Olive green to dark forest green with lighter green or white phenocrysts; Luster: Dull to waxy when wet; Crystal structure: Porphyritic (large crystals in a fine-grained matrix); Cleavage: Indistinct; Specific gravity: 2.8-3.0.
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to waxy when wet
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Olive green to dark forest green with lighter green or white phenocrysts; Luster: Dull to waxy when wet; Crystal structure: Porphyritic (large crystals in a fine-grained matrix); Cleavage: Indistinct; Specific gravity: 2.8-3.0.
Formation & geological history
Formed through volcanic activity where magma cooled in two stages: first slowly deep underground to form large crystals, then rapidly during eruption. The green color comes from low-grade metamorphism (greenschist facies) which replaces original minerals with chlorite, epidote, and actinolite.
Uses & applications
Used historically for stone tools and amulets; currently used as decorative gravel, in landscaping, and as a semi-precious stone for lapidary work and river rock polishing.
Geological facts
Many greenstones found on beaches and in riverbeds are ancient volcanic rocks that have been metamorphosed over millions of years. The specific 'spotted' appearance is often referred to as 'Chinese Writing Stone' in certain lapidary circles if the crystals are needle-like.
Field identification & locations
Identify by the distinct 'porphyritic' texture (visible spots or crystals) and the dark green hue. Commonly found in areas with ancient volcanic arcs or tectonic subduction zones, such as the Pacific Northwest or the Great Lakes region.
