Slate is a fine-grained, low-grade metamorphic rock, derived from mudstone or occasionally volcanic ash. The essential minerals present in all slates are quartz and the platy minerals; chlorite and white mica, however the relative proportion of the different minerals affects the durability of the material.
Slate is characterised by its slaty cleavage formed during metamorphism by the alignment of platy minerals into parallel planes. This enables the rock to be split into thin sheets, making it suitable as a roofing material. The proportion of platy minerals present and their size affect the type of cleavage and hence its ability to be split into slabs of suitable thickness.
Other rock types, often referred to as slate when used as roofing material, include phyllite, mica schist and flagstone. Phyllite is similar to slate in that it is split along a pervasive slaty cleavage, however it is coarser-grained due to more intense metamorphism giving it a slight sheen. Ballachulish slate is an example of a phyllite although traditionally it has always been referred to as slate. Cnocfergan slate is another rock referred to as slate because of its use as a roofing material. In this case the rock is a mica schist, which apart from being a metamorphic rock is very dissimilar from slate. It is very coarse-grained and can only be split along micaceous layers which are primary bedding features. Flagstone, such as Caithness, is a sedimentary rock split along bedding.