Slate is a fine-grained metamorphic rock It can be black, grey, bluish, purplish or greenish grey. Slate contains many smooth, thin, flat layers (foliation) and it splits easily into sheets.
How is it formed?
Slate is usually formed from mudstone that has been put under pressure and heated up during plate collisions and mountain building. Pressure causes the platy clay minerals to line up parallel to each other and so the rock splits easily into sheets.