This specimen is from Bryn y Rafr mine, on the shore of Nant y Moch reservoir, West Wales. It is a loose breccia, with a number of voids, and is typical from this mine. It has components of quartz, marcasite, and wall rock. The quartz is very fragile, and the marcasite is suffering from pyrite decay, which is leaving a deposit of sulphur. The marcasite is of a granular nature as seen in the inset picture. The pyrite decay is occurring, even though the specimen is kept in a sealed box, with a considerable number of dessicant sachets.