The rotten stone is a unit of sandy limestone which occurs at the contact between the Carboniferous Limestone and Millstone Grit on Mynydd Du.
When weathered this rock has the weight of pumice and crushes easily to powder. The powdered rock was widely used as as an abrasive and as a polish in the copper and tinplate industries of South Wales. The rotten stone was extracted from shallow pits and transported by sledges or tramroads.
The remains of an early opencasting method of working these deposits can be seen on the hillside north of Cribarth where a triangular patch of land is pocked with innumerable small hollows. There are further abandoned workings north of Cwmllynfell on the slopes of Cefn Carn Fadog.
Working of rotten stone had ceased by the 1930s.