Slate quarries, Harlech Castle. 18th-century artwork of slate being quarried (lower left) near Harlech Castle. Slate is a grey rock that can be split into thin, flat sheets. It is quarried for use in the construction industry. The slate industry in Wales grew from small quarries to a large industry in the 19th century. Harlech Castle was built between 1283 and 1290 as part of the fortifications constructed after King Edward I of England invaded North Wales in 1282. It is located close to the Irish Sea coast. Dating from 1798, this artwork is by British landscape painter John Hassell (1767-1825).