Greenstone Identification Guide
A field guide to identifying greenstone, an altered massive mafic volcanic rock, by its green color, toughness, and lack of foliation.
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What Greenstone Looks Like
Greenstone is a general field term for altered (metamorphosed) mafic volcanic rock—typically metabasalt—whose green color comes from secondary minerals like chlorite, epidote, and actinolite that replaced the original dark minerals. It is massive (non-foliated), fine-grained, dense, and tough, ranging from dull olive to dark green, with a dull-to-slightly-greasy luster. Relict volcanic features such as pillow structures or amygdules (filled gas cavities) may survive.
Step-by-Step Field-ID Checklist
- Confirm the green color throughout the rock, not just on weathered surfaces.
- Check for massive texture: no splitting layers (unlike greenschist).
- Feel the toughness and density: greenstone is hard to break and feels heavy.
- Look for relict volcanic features: pillows, amygdules, or fine grain hinting at lava origin.
- Scratch test: the rock is generally hard, though soft chlorite patches may scratch.
Key Diagnostic Tests
- Mohs hardness: overall hard (~5.5–6.5); soft chlorite zones lower
- Streak: pale greenish gray
- Fabric: massive, non-foliated (key contrast with greenschist)
- Specific gravity: ~2.9–3.1 (dense, mafic)
- No strong acid reaction unless carbonate-altered (some greenstones contain calcite amygdules that fizz)
- May contain minor magnetite giving slight magnetic response
Common Look-Alikes and How to Tell Them Apart
- Greenschist: Same mineralogy and color but foliated/schistose; greenstone is massive without splitting layers.
- Basalt (fresh): Black/dark gray and unaltered; greenstone is its green, chlorite-altered equivalent.
- Serpentinite: Soapier, smoother, lower density in places, derived from ultramafic rock; greenstone derives from basalt.
- Nephrite jade: Translucent on edges, far tougher and more uniform; greenstone is opaque and rock-like. (Note: in New Zealand "greenstone" colloquially means nephrite/pounamu—context matters.)
- Epidosite: Dominated by epidote, brighter pistachio-green and granular.
Where Greenstone Is Found
Greenstone is the building block of Archean greenstone belts—ancient volcanic-sedimentary sequences that host major gold and base-metal deposits—found in cratons of Canada, Australia, southern Africa, and elsewhere. It also occurs in younger metamorphosed volcanic terranes worldwide.
Frequently asked questions
How can you tell if it's greenstone?
Greenstone is a dense, tough, massive green rock without foliation, colored by chlorite, epidote, and actinolite, and it often preserves volcanic features like pillows or filled gas cavities.
What is the difference between greenstone and greenschist?
They share the same green minerals, but greenstone is massive and does not split, while greenschist is foliated and splits along aligned mineral layers.
What does greenstone look like?
It is a fine-grained, dull olive-to-dark-green rock that is hard and heavy, sometimes showing relict pillow lava shapes or amygdules from its volcanic origin.
Is greenstone the same as jade?
Not in geology; geologic greenstone is altered basalt, but in New Zealand 'greenstone' is a common name for nephrite jade (pounamu), so the meaning depends on context.
What is a greenstone belt?
A greenstone belt is an ancient, often Archean, sequence of metamorphosed mafic volcanic and sedimentary rocks that frequently hosts important gold and base-metal deposits.
Greenstone identified by the community
Recent Greenstone specimens identified with Rock Identifier.
