igneous

Greenstone (Amygdaloidal Basalt)

Amygdaloidal Basalt (frequently composed of Plagioclase Feldspar, Pyroxene, and Olivine with Chlorite inclusions)

Hardness: 5-7 (Mohs scale); Color: Dark forest green to grey-green; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained) with amygdules; Cleavage: None/Irregular fracture; Specific Gravity: 2.8 - 3.0

Hardness: 5-7 (Mohs scale)
Color: Dark forest green to grey-green
Luster: Dull to sub-vitreous

Identified March 21, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the cooling of mafic lava flows where gas bubbles (vesicles) were later filled with secondary minerals like chlorite, epidote, or quartz through hydrothermal activity. Often associated with Precambrian volcanic sequences.

Uses & applications

Used primarily as decorative lawn stone, crushed aggregate for construction, and occasionally in lapidary work for making 'Lake Superior greenstone' jewelry (if containing Chlorastrolite).

Geological facts

This rock is a classic example of low-grade metamorphism where the original basaltic minerals have been altered to green minerals like chlorite. The 'amygdules' are the small, light-colored circular spots which were originally gas bubbles in the lava.

Field identification & locations

Identify in the field by its distinctive 'bumpy' or 'spotted' appearance (amygdules) and dark green hue. Frequently found as river or beach pebbles in the Great Lakes region of North America and other ancient volcanic terrains.

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