Diphwys Casson Quarry
Diphwys Casson Quarry (sometimes known as Diphwys Quarry or Diffwys Quarry) was a major slate quarry in the town of Blaenau Ffestiniog, north Wales. It was one of the major users of the Ffestiniog Railway.
To the north east of the town of Blaenau Ffestiniog is the valley of Cenunant y Diphwys. In the 1760s men led by Methusalem Jones, from the long established Cilgwyn quarry near Nantlle started quarrying in this area, which was known for its slate beds. The exact location of this original quarry has been obliterated by subsequent mining activity, but it is likely that it was part of what became Diphwys Casson. In 1800 quarry managers William Turner and William Casson, from the Lake District took the lease on the quarry. They substantially expanded the workings. By 1820, underground mining had started. 
The quarry was successful in the period 1820–1880, but it ran into difficulties and production ceased in 1890. Working restarted in 1920, but this lasted only seven years before the quarry was again abandoned.
Diphwys Casson closed in 1955 after almost two centuries of operation.
- Gwynfor Pierce Jones and Dafydd Walter Dafis (2002). "Water Power in the Slate Mines of East Ffestiniog" (PDF).
- "Diphwys Quarry, Blaenau Ffestiniog".
- Richards, Alun John (1995). Slate Quarrying in Wales'. Gwasg Carreg Gwalch. ISBN 0-86381-319-4.