Igneous

Porphyritic Basalt

Amygdaloidal Porphyritic Basalt

Hardness: 6 (matrix) to 6.5 (phenocrysts). Color: Dark grey to black groundmass with white or light cream phenocrysts. Luster: Dull/earthy for groundmass, vitreous for crystals. Structure: Aphanitic matrix with visible large crystals (phenocrysts). Cleavage: None for groundmass;…

Hardness: 6 (matrix) to 6
Luster: Dull/earthy for groundmass, vitreous for crystals

Identified March 11, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of basaltic lava at the Earth's surface or shallow depth. The large white crystals (likely Plagioclase Feldspar) began growing slowly deep underground before the entire magma mass was erupted, causing the remaining liquid to cool quickly and create the fine-grained dark background.

Uses & applications

Used primarily as road ballast, crushed stone for construction, and decorative landscaping rock. Sometimes polished for aquarium use or used in geological studies to understand volcanic history.

Geological facts

This specific textural pattern is often nicknamed 'Starry Night Rock' or 'Chinese Writing Stone' in certain varieties. It represents two distinct cooling stages in a single volcanic event. The rounded shape indicates it has been weathered by water movement, likely in a river or coastal environment.

Field identification & locations

Identify in the field by the 'porphyritic texture' (cookies-and-cream appearance) and the presence of small gas bubbles (vesicles) or filled gas bubbles (amygdules). Common in volcanic regions such as the Pacific Northwest of the US, the Lake Superior region, and Iceland.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

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