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If South Wales is synonymous with coal or 'black gold', then North Wales has been equally blessed and cursed with its equivalent - slate.

Slate is formed when mud with high levels of clay was compacted and squeezed at high temperature during continental shifts some 300-400 million years ago. Depending on the specific content and age of the slate, it is assigned grades of quality, and the seams of slate around Dinorwig, Llanberis, Bethesda and Blaenau Ffestiniog are some of the highest quality in the world.

It was the Romans who first took slate from the area to use for buildings at Segontium, or Caernarfon as it's known today, and the material was also used by the English castle builders of the medieval period.

It was at the end of the 18th century, however, that demand for the material exploded, changing the look and life of North West Wales forever.

Welsh slate was in demand not only in Britain but also in North America and mainland Europe as industrialisation and populations across the world gathered pace. Mines at Dinorwig, Penrhyn, Llanberis, Bethesda and Blaenau Ffestiniog opened or expanded hugely to cater for demand that increased rapidly.

By the 1870s, slate was one of Wales' major industries and Blaenau Ffestiniog - previously a name for the towering mountains overlooking the beginnings of a small river valley - had become an industrial town. The Dinorwig and Penrhyn quarries were the two largest slate mines in the world, each employing over 3000 people.

Infrastructure developed to cater for the industry and facilitate its transportation around the world. The narrow-gauge railway from the Llechwedd complex at Blaenau Ffestiniog to the new port of Porthmadog was one such scheme. It is now a world-famous tourist attraction.

The main, standard gauge railway network caught up and linked all the main quarry sites to the rest of Great Britain by rail.

But, in common with other examples of heavy industries, slate mining came with iniquities, trials, hardships and danger.

Men worked six days a week for very poor wages and in sometimes desperate conditions. It took five years of apprenticeship to become a full miner, and even then the skilled labour was not reflected in pay. They were employed on monthly contracts, with the first three weeks of each month being paid in 'sub' wages: a nominal subsistence amount.

The fourth week was paid with profits and bonuses added, less cost of... everything, essentially. Miners had to pay for explosives, tools, sharpening and even air for the pneumatic drills. It didn't add much to the paypacket.

Physical conditions, too, were harsh. Skilfully wielding the hammers and chisels that were the tools of their trade, they would dangle on ropes round their body and legs from pins at the top of the gallery in which they were working. The slate was nearly always damp and slippery, there could be rock falls and the ropes often broke.

If an accident happened, the on-site medical facility was a St John's Ambulance station run by miners themselves as volunteers. For men working away from the seam, slate dust was the major cause of medical complaint. This could - like coal dust for miners in South Wales - cause silicosis and other respiratory illnesses.

The pace with which the slate mining industry expanded during the 19th century was such that although the men worked extremely hard, demand outstripped production and the UK market turned to Spanish imports to take up the slack.

In 1900 the quarrymen at Dinorwig went on strike over pay, conditions and union representation. The dispute lasted for three years and precipitated a decline in the industry that carried on throughout the 20th century.

The market for Welsh slate contracted as other materials began to be used for roofing, and in the latter half of the 20th century onwards the material has had increasingly specialised uses such as snooker tables and ornaments. Dinorwig closed the day after supplying the dais used for Prince Charles' investiture as Prince of Wales in 1969.

These days, sites at Llanberis and Blaenau Ffestiniog remain open as tourist attractions, while Penrhyn mine is a going concern.