Blueschist is a distinctive rock that forms by the metamorphism of sodium-rich basic rocks at high pressures and low temperatures. Conditions are good for that in subduction, where marine crust and sediments are subducted beneath a continental plate and kneaded by changing tectonic motions while sodium-rich fluids marinate the rocks. It's a schist because all traces of original structure in the rock have been wiped out along with the original minerals, and strongly layered fabric has been imposed.
Petrologists often prefer to talk about the glaucophane-schist metamorphic facies rather than blueschist, because not all blueschist is all that blue. In this hand specimen, from Ward Creek, California, glaucophane is the major blue mineral species, but in other examples lawsonite, jadeite, epidote, phengite, garnet, and quartz are also common. It depends on the original rock that is metamorphosed. The bluest, most schistose blueschist is made from mafic rocks like basalt and gabbro.
As a landscaping stone, blueschist is responsible for some striking, even garish effects.
For more photos see the Metamorphic Rocks Gallery.
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