Argillite does have a glamorous side that slate can't match.It is also known as pipestone when it lends itself to carving.In other samples, lawsonite, jadeite, epidote, phengite, garnet, and quartz are also common.It depends on the original rock that is metamorphosed.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.It was part of a subducting plate during Jurassic times, about 170 million years ago, when it formed.During the last few million years, it was raised and mixed into younger subducted rocks of the Franciscan complex.
This type of metamorphic was created by regional metamorphism, in which a sedimentary or igneous rock has been deeply buried and subjected to high temperatures and pressures.
For instance, a blueschist-facies ultramafic rock consists mainly of serpentine (antigorite), olivine and magnetite.
Eclogite ("ECK-lo-jite") is an extreme metamorphic rock formed by regional metamorphism of basalt under very high pressures and temperatures.
The American Indians favored it for tobacco pipes and other small ceremonial or decorative objects.
High-pressure, low-temperature conditions are most typical of subduction, where marine crust and sediments are carried beneath a continental plate and kneaded by changing tectonic motions while sodium-rich fluids marinate the rocks.