igneous
Porphyritic Rhyolite (Cookie Stone)
Porphyritic Rhyolite (SiO2, with phenocrysts)
Hardness: 6-7 (Mohs scale); Color: Pinkish-red groundmass with dark black/grey spots; Luster: Dull to vitreous; Crystal Structure: Aphanitic (fine-grained) groundmass with visible Phenocrysts; Cleavage: None (conchoidal fracture); Specific Gravity: 2.4 - 2.6
- Hardness
- 6-7 (Mohs scale)
- Color
- Pinkish-red groundmass with dark black/grey spots
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Pinkish-red groundmass with dark black/grey spots; Luster: Dull to vitreous; Crystal Structure: Aphanitic (fine-grained) groundmass with visible Phenocrysts; Cleavage: None (conchoidal fracture); Specific Gravity: 2.4 - 2.6
Formation & geological history
Formed from the rapid cooling of silica-rich lava at or near the Earth's surface. The larger dark crystals (phenocrysts) formed slowly underground before being caught in a volcanic eruption that cooled the rest of the rock quickly. These typically originate from the Proterozoic to Cenozoic eras.
Uses & applications
Used primarily as decorative gravel, in landscaping, as a construction aggregate, and as a novelty 'pocket stone' for rock collectors due to its resemblance to a chocolate chip cookie.
Geological facts
This specific appearance is often nicknamed the 'Cookie Stone' or 'Chocolate Chip Rock.' It is high in silica, often exceeding 70%, making it the volcanic equivalent of granite.
Field identification & locations
Identify in the field by its distinctive 'spotted' appearance where dark crystals are embedded in a light, fine-grained matrix. Commonly found in volcanic regions such as the American Southwest (Arizona, New Mexico) and parts of the Great Lakes region as glacial till.
