The Diffwys (also known as Casson) Slate Quarry (CN413; GA HER PRN 20305; RCAHMW NPRN 85487), part of the Slate Landscape of Northwest Wales World Heritage Site, is located immediately NE of Blaenau Ffestiniog, Gwynedd. It opened in the 1760s and was the first formalised quarry in the town. It was known as the "mam-chwarel" (mother quarry) of Ffestiniog, the town’s primary quarry and the first to export slate to markets beyond North Wales. The quarry achieved peak output between the 1820s and 1880s during which time it produced over 5000 tons of slate products per annum and employed over 200 men. Production at the quarry declined towards the end of the 19th century and the company that owned it was declared bankrupt in 1892. Small-scale extraction continued at the site until 1925.

Melissa Lambe and Neil McGuinness of Heneb: Gwynedd Archaeology undertook a photogrammetric and laser scan survey of some of the Floor 6 structures at the quarry on behalf of Cadw as part of condition monitoring work in March 2024.