Diphwys Casson is the oldest of the major quarries in the Blaenau Ffestiniog area. It was started in the 1760s by Methusalem Jones, who was apparently divinely instructed in a dream to come and dig in this area, and finished up making a fortune. By the 1820s it produced some 6000 tons of finished slate each year. In the 1880s it ran into trouble. In attempt to restore its fortune the New Workings quarry was opened. The quarry was idle between 1890 and 1920 and again from 1927 onwards.
The Diphwys quarry up to 1800 only gave employment to 20 men; in that year the quarry and farm on which it was situated were put up for auction. William Turner, who had been working a small quarry near Llanrwst, sent for two of his Lancashire friends, Messrs. Thomas and William Casson, and by pooling their resources bought the property for £1,000. They had no capital to work the quarry and so they took into partnership Hugh Jones, a wealthy banker from Dolgelly, who was also interested in the Dinorwic Slate Company. The slate vein proved to be very valuable and was near the surface so that the cost of removing topsoil and unproductive rock was not excessive. The Company secured lucrative Government contracts to roof barracks at Portsmouth, Plymouth, Dublin, Cork and other places. William Turner was the active partner and kept all the accounts; all monetary transactions was carried out through the firm of Thornas and Hugh Jones, Bankers, Dolgelly.
The Diphwys quarry proved very remunerative. Between 1809 and 1828 Turner acted as general manager of the Dinorwic quarry; in the 'twenties he was an active partner in small companies working the Penybryn and Penyrorsedd quarries in the Nantlle district and the Cloddfa'r Coed quarry in the Bethesda district.
Evan Parry Jones managed the Diphwys Casson Slate Co. He married Jane, daughter of John Vaughan of Tanymanod. In 1893 they moved to Blaenddôl. By 1905, Jane Parry Jones had been widowed, and she herself died in about 1927, although Blaenddôl continued to be occupied by her daughters.
Slate Quarries in Wales