Slate from Grythyttan

Grythyttan has the oldest slate deposits of Sweden. They were formed approximately 1.9 billion years ago at the bottom of the sea. The material is termed primeval clay slate. The slate is either gray or black depending on which quarry it originates from. It derives from organic material, and it's mixed in cyanobacteria that has given it the colour.

The slate was manufactured in roughly 40 different sizes and were always measured in inches, which is the unit of measurement used in slate roofing even today. The production of roof slate has started again, after being closed down since 1973. The black slate, called Stentry after the Stentry quarry, is the kind produced today.