igneous
Greenstone / Diorite
Intermediate plutonic igneous rock
Hardness: 6-7 (Mohs scale), Color: Dull grayish-green with salt-and-pepper salt textures, Luster: Sub-vitreous to dull, Crystal structure: Phaneritic (medium-grained), Specific gravity: 2.8-3.0
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Dull grayish-green with salt-and-pepper salt textures, Luster: Sub-vitreous to dull, Crystal structure: Phaneritic (medium-grained), Specific gravity: 2.8-3.0
Formation & geological history
Formed through the slow cooling of magma under the Earth's surface, typically at volcanic arches near subduction zones or in mountain-forming regions (orogenic belts). The green tint is often due to low-grade metamorphism or hydrothermal alteration of mafic minerals into chlorite or epidote.
Uses & applications
Commonly used in construction as crushed stone or road base. Dense, water-worn specimens are popular for rock tumbling and as decorative garden stones.
Geological facts
Many 'greenstones' were used by prehistoric cultures to create 'ground stone' tools like axes and celts because of their toughness and resistance to shattering.
Field identification & locations
Identify in the field by its characteristic 'salt and pepper' look where dark and light minerals are evenly mixed. Often found in riverbeds or coastal areas where erosion has smoothed the rock into pebbles.
