igneous

Basalt Porphyry

Porphyritic Basalt

Hardness: 6 (Mohs scale); Color: Dark grey to black groundmass with light-colored (white or cream) phenocrysts; Luster: Dull or sub-metallic; Crystal structure: Aphanitic (fine-grained) matrix with visible large macro-crystals; Cleavage: Generally poor; Specific Gravity: 2.8–3.0

Hardness: 6 (Mohs scale)
Luster: Dull or sub-metallic

Identified March 20, 2026More igneous →

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Physical properties

Formation & geological history

Formed from volcanic activity where magma begins cooling slowly underground, allowing large crystals (phenocrysts) to form, followed by a rapid eruption and quick cooling of the remaining molten material. Found in lava flows from the Cenozoic to the Precambrian eras.

Uses & applications

Used primarily in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Distinctive specimens are prized by rockhounds and used for ornamental paving or decorative garden stones.

Geological facts

The term 'porphyry' comes from the Ancient Greek word for purple, as the first specimens described were a deep purple color with white crystals. This rock specifically demonstrates two distinct cooling rates in a single volcanic event.

Field identification & locations

Identify in the field by its 'chocolate chip cookie' appearance: a dark, dense, heavy basaltic background with light-colored, angular crystals embedded within. Commonly found in volcanic regions such as the Pacific Northwest, Hawaii, 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