Slate quarry. View of the Penrhyn slate quarry in Bethesda, North Wales. Newly quarried slate is transported to the conveyor belts (centre left), which carry it to the processing and storage equipment (centre). Slate is a mineral that is naturally formed in thin, hard layers, making it ideal for use as tiles or flagstones. This quarry is one of the largest in the world. It is well over 200 hectares in size, and goes down over 350 metres. Excavations started at this quarry in 1765.