Graig-ddu slate quarry lies to the east of Blaenau Ffestiniog in a remote, high altitude (>600m) location. It is almost entirely given over to open quarrying, the only such example in the Ffestiniog area. The quarry opened in about 1800, as the Manod quarry, developing as Graig-ddu in the 1840s. Its location raised considerable transport difficulties from the outset, until the opening of the Ffestiniog & Blaenau Railway in 1868, and because of water supply problems the mill had to be sited below the quarry level, powered by an entirely buried wheel fed by an iron-lined underground leat. In much later mill developments, gas power was employed but the quarry was never electrified. One mill was converted to a barracks.
Although output was only some 3000 tons per annum the workings were reported to have 36 saw-tables and 36 dressing machines on site. The quarry was notable for its unique use of 'wild cars' (car gwyllt) which conveyed men down from the quarry after work, and in use until closure. Quarry output peaked at 3140 tons in 1882 with 110 men. In 1898 218 men were employed, dwindling to 86 in 1937-8. Operations ceased in the 1940s though small scale sporadic work continued. In the 1980s untopping work commenced in conjunction with the New Manod quarry (NPRN 40598). This work removed much of the surviving quarry infra-structure. What survives (in 1991) are tramway formations, the vestiges of some small buildings, and an incline.

Sources:
D.Gwyn, Welsh Slate: the Archaeology and History of an Industry (RCAHMW 2015), 137.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), 130-1.

David Leighton, RCAHMW, 11 February 2015