igneous
Green Andesite (or Greenstone / Altered Volcanic Rock)
Intermediate Volcanic Rock with Chlorite/Epidote Alteration
Hardness: 5.5-6.5 Mohs. Color: Sea green to pale muted green with dark and light phenocryst/clast speckles. Luster: Dull to earthy/sub-vitreous. Texture: Aphanitic to fine-grained porphyritic, naturally water-worn and smoothed. Specific Gravity: ~2.6-2.8.
- Hardness
- 5
- Luster
- Dull to earthy/sub-vitreous
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Physical properties
Hardness: 5.5-6.5 Mohs. Color: Sea green to pale muted green with dark and light phenocryst/clast speckles. Luster: Dull to earthy/sub-vitreous. Texture: Aphanitic to fine-grained porphyritic, naturally water-worn and smoothed. Specific Gravity: ~2.6-2.8.
Formation & geological history
Formed via extrusive volcanic activity where silica-intermediate lava (or volcanic tuff/breccia) cooled and was subsequently subjected to low-grade hydrothermal metamorphism, converting primary ferromagnesian minerals into green secondary minerals like chlorite, epidote, or actinolite. Later eroded and smoothed by stream or beach wave action.
Uses & applications
Primarily collected as a decorative beach pebble, tumbling rock, or lapidary material for cabochons and small ornamental carvings. Historically, tough greenstones were used to make stone tools like adzes and axes.
Geological facts
Green-tinted volcanic pebbles are often colloquially referred to as 'greenstone' or 'chloritized volcanic rocks'. Their distinct greenish hue comes from secondary alteration minerals, turning originally dark mafic or intermediate volcanic rocks green over geological time.
Field identification & locations
Identified in the field by its distinct pale-to-medium green color, smooth water-rounded oval shape, and mottled speckled texture consisting of fine mineral grains and small phenocrysts. Commonly found along volcanic coastline beaches, riverbeds, and gravel deposits worldwide.
