Slate

Slate, a fine-grained, homogeneous, sedimentary rock being composed of clay or volcanic ash that has been metamorphosed in layers. It is also made into roofing slates, called 'roofing shingles' in the US, due to its two lines of breakability, the cleavage and the grain. This property of the slate makes it possible to split it into thin sheets. Finer slates are also used to hone knives.

The composition of slate is mainly of quartz and muscovite (a mica), often along with biotite, chlorite, and hematite, or, less frequently, apatite, graphite, magnetite, kaolin, tournaline, or zircon.

The finer quality of slates come from the Delabole in Cornwall, Portugal, Wales in the UK, the east coast of Newfounland, and of course the "Slate Valley" of Vermont and New York.

Slates were also found in the Arctic and it was used by the Inuit to make the blade for ulu's