Blueschist
“Crushed at the bottom of the sea”
Metamorphic
Blueschist is a rare metamorphic rock formed under conditions of high pressure but relatively low temperature — a combination that typically occurs only in subduction zones. Its distinctive blue color comes from the mineral glaucophane.
Composition
glaucophane (blue amphibole), lawsonite, epidote, garnet, jadeite
Where Found
Franciscan Complex (California, USA), Cyclades (Greece), Western Alps, Japan
Mohs Hardness
5-7
Fun Fact
Blueschist can only form under very specific conditions, so finding it on a geological map is like finding a fossil subduction zone — proof that tectonic plates once collided there.
Test Your Knowledge
Why is blueschist virtually absent from rocks older than about 1 billion years?
Related Specimens
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