Slate is a fine-grained, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism with fine cleavage planes.

When expertly “cut” by striking parallel to the cleavage plane with a specialized tool in the quarry, many slates will form smooth flat sheets of stone which have long been used for roofing, floor tiles, and other purposes. Slate is frequently grey in colour, especially when seen, en masse, covering roofs. However, slate occurs in a variety of colours even from a single locality; for example, our slate from Snowdonia can be found in many shades of grey, from pale to dark, and may also be purple, green or cyan.