igneous
Porphyritic Basalt
Porphyritic Basalt with Olivine phenocrysts
Hardness: 6 (basalt matrix), 6.5-7 (olivine); Color: Dark grey to black groundmass with light green to yellow-green phenocrysts; Luster: Dull/earthy matrix with glassy/vitreous crystals; Structure: Porphyritic (fine-grained matrix with larger visible crystals); Specific Gravity: 2.8–3.0
- Hardness
- 6 (basalt matrix), 6
- Luster
- Dull/earthy matrix with glassy/vitreous crystals
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Physical properties
Hardness: 6 (basalt matrix), 6.5-7 (olivine); Color: Dark grey to black groundmass with light green to yellow-green phenocrysts; Luster: Dull/earthy matrix with glassy/vitreous crystals; Structure: Porphyritic (fine-grained matrix with larger visible crystals); Specific Gravity: 2.8–3.0
Formation & geological history
Formed from the cooling of lava in two stages. Large olivine crystals (phenocrysts) formed slowly underground before a volcanic eruption rapidly cooled the remaining liquid magma into a fine-grained dark matrix.
Uses & applications
Used in construction, as road aggregate, in landscaping, and as a geological specimen for studying volcanic processes.
Geological facts
The green crystals are often Olivine (Peridot). This specific texture is called 'Porphyritic,' which creates a 'starry' or 'snowflake' appearance depending on the mineral shape. Similar rocks found in the Lake Superior region are known as 'Chinese Writing Stone' or 'Starfish Stone' when the minerals vary.
Field identification & locations
Identify in the field by searching for dark, dense volcanic rocks containing distinct light green glassy grains. Commonly found in volcanic landscapes like Hawaii, Iceland, or ancient lava flows in the Pacific Northwest.
