igneous
Porphyritic Basalt (Peanut Obsidian/Star Galaxy-style)
Porphyritic Basalt (Silica-rich Volcanic Rock)
Hardness: 6-7 on Mohs scale. Color: Dark grey to black groundmass with white/off-white phenocrysts (crystals). Luster: Dull to sub-vitreous. Structure: Aphanitic matrix with visible large crystals (porphyritic texture). Specific Gravity: Approximately 2.8 - 3.0.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Dark grey to black groundmass with white/off-white phenocrysts (crystals). Luster: Dull to sub-vitreous. Structure: Aphanitic matrix with visible large crystals (porphyritic texture). Specific Gravity: Approximately 2.8 - 3.0.
Formation & geological history
Formed from rapid cooling of volcanic lava that already contained larger, pre-formed crystals grown slowly deep underground. This specimen likely dates from the Tertiary or Cretaceous periods, often found in volcanic fields or alluvial deposits.
Uses & applications
Primarily used as decorative stone, lapidary material for cabochons or beads, and as architectural stone (countertops/tiles in similar variants like Star Galaxy granite).
Geological facts
This texture is known as 'porphyritic,' where two distinct stages of cooling occurred: a slow stage underground forming the large white crystals (phenocrysts), followed by a rapid eruption that froze the rest of the lava into a fine-grained black matrix.
Field identification & locations
Identify in the field by the 'chocolate chip' or 'peanut' appearance where light crystals stand out sharply against a dark, fine-grained background. Found frequently in the American Southwest (Arizona, New Mexico) and volcanic regions of Mexico.
