 The remains of the main Diphwys Casson mill | |
| Location | |
|---|---|
  Diphwys Casson quarry Location in Gwynedd | |
| Location | Blaenau Ffestiniog |
| County | Gwynedd |
| Country | Wales |
| Coordinates | 52°59′53″N 3°54′58″W / 52.998°N 3.916°W grid reference SH7142946270 |
| Production | |
| Products | Slate |
| Type | Quarry |
| History | |
| Discovered | 1760s |
| Opened | 1800 |
| Active | 1800-1890; 1920-1927 |
| Closed | 1955 |
Diphwys Casson quarry (sometimes known as Diphwys quarry or Diffwys quarry) was a major slate quarry in the town of Blaenau Ffestiniog, Gwynedd, North Wales. It was one of the major users of the Ffestiniog Railway.