igneous

Basalt Porphyry (River Pebble)

Porphyritic Basalt

Hardness: 6 (Mohs scale); Color: Dark grey to charcoal with light-colored phenocrysts; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained) groundmass with visible macro-crystals (plagioclase feldspar); Specific gravity: 2.8–3.0.

Hardness: 6 (Mohs scale)
Color: Dark grey to charcoal with light-colored phenocrysts
Luster: Dull/Earthy

Identified February 10, 2026More igneous →

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Physical properties

Formation & geological history

Formed through rapid cooling of mafic lava on the Earth's surface. The larger white spots (phenocrysts) formed earlier at depth through slower cooling before the final eruption. This specific specimen has been rounded and smoothed by water erosion in a river or coastal environment.

Uses & applications

Used in construction as crushed stone or aggregate. Historically used for paving stones. Smooth pebbles are popular in landscaping, aquariums, and rock tumbling.

Geological facts

Basalt is the most common rock in the Earth's crust, making up most of the ocean floor and large volcanic plateaus. Porphyritic textures tell a 'two-stage' cooling story of the volcano's history.

Field identification & locations

Identify in the field by its dark, dense matrix and the presence of lighter, blocky crystals. Commonly found in volcanic regions and transported by rivers. Collectors look for high-contrast 'snowflake' patterns.

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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

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Magnetite

Magnetite (Fe3O4)

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