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

Identified March 26, 2026More igneous →

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Physical properties

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.

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Fluorapatite (Ca5(PO4)3F)

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Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

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Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral