igneous
Vesicular Basalt with Epidote/Amygdaloidal Basalt
Amygdaloidal Basalt (Mg, Fe)2SiO4 + Ca2(Al, Fe)3(SiO4)3(OH)
Hardness: 6-7 (Mohs). Color: Dark green to grey-green (basalt) with bright pistachio green spots (epidote). Luster: Dull to vitreous. Crystal structure: Microcrystalline groundmass with amygdaloidal fillings. Specific gravity: 2.8 - 3.2.
- Hardness
- 6-7 (Mohs)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs). Color: Dark green to grey-green (basalt) with bright pistachio green spots (epidote). Luster: Dull to vitreous. Crystal structure: Microcrystalline groundmass with amygdaloidal fillings. Specific gravity: 2.8 - 3.2.
Formation & geological history
Formed from the cooling of mafic lava flows where gas bubbles (vesicles) were later filled with secondary minerals like epidote, chlorite, or quartz through hydrothermal activity. Common in ancient volcanic terrains like the Keweenaw Peninsula.
Uses & applications
Primarily used for lapidary purposes, decorative stones, and as a geological specimen. Sometimes used in road construction as crushed stone.
Geological facts
The green spots are often Epidote, a mineral that forms during low-grade metamorphism. These rocks are sometimes mistakenly called 'puddingstones' by locals, though they are volcanic rather than sedimentary.
Field identification & locations
Identify by the dark, fine-grained matrix containing distinctive rounded green mineral 'pockets.' Commonly found in the Lake Superior region and the Pacific Northwest. Collectors should look for high contrast between the matrix and the fillings.
