What is slate? 

The word slate is derived from the late Middle English slat or sclate. This is in turn connected with the French verb esclater meaning to break into pieces. This is an allusion to the splitting of slates into thin sheets. In Welsh, all fissile rocks can be referred to as llech but the older Welsh name ysglatus, ysglats, sglaets or sglatys was used exclusively for slate. 

In Wales, slate of economic value is contained in Gwynedd, Denbighshire, Montgomery and Pembrokeshire, even though some of uneconomic value is also to be found in counties like Anglesey. Slate producing areas are in Cornwall, in Devon and the Lake District in England and in Aberdeenshire and Argyll in Scotland.