Igneous

Greenstone / Diabase Porphyry

Porphyritic Diabase (Micro-gabbro)

Hardness: 6-7 (Mohs). Color: Dark green to grey-green matrix with darker green phenocrysts. Luster: Dull to waxy when wet/polished. Structure: Fine-grained matrix with larger, visible crystal clusters (porphyritic texture). Specific gravity: 2.8 - 3.2.

Hardness: 6-7 (Mohs)
Luster: Dull to waxy when wet/polished

Identified April 7, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the slow cooling of mafic magma (basaltic composition) within the Earth's crust, typically in dikes or sills. The distinct green color is often the result of low-grade metamorphism (greenschist facies) where minerals like chlorite, epidote, or actinolite replace primary minerals.

Uses & applications

Used primarily in construction as crushed stone or road gravel. Polished specimens are used in lapidary work, decorative landscaping, and collecting. Historically used for making stone tools due to its toughness.

Geological facts

Greenstone is a general field term for any dark, greenish-colored igneous rock. The darker spots in this specimen are likely concentrations of augite or hornblende crystals. Ancient cultures often favored greenstones for ceremonial axes.

Field identification & locations

Identify in the field by its characteristic 'dull' green hue and high density. It lacks the layering of metamorphic slate and is harder than most sedimentary rocks. Commonly found in areas of ancient volcanic activity such as the Lake Superior region or the Appalachian Mountains.

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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral