Slate is a hard rock that is made when clay is put under enormous pressure, often when new mountains are being formed. At this time all of the tiny clay crystals are turned into the same direction. This is what makes a slate break in sheets.

If the edge of a piece of slate is struck, the layers separate and thin sheets of rock with very smooth surfaces are formed. These sheets are waterproof and can be used on a roof.

The smooth, hard surface of slate makes it a useful material in making billiards, pool and snooker tables. It forms the base on which the green (felt) cloth is laid.