There are many badly formed prismatic crystals of quartz, SiO2 lining this cavity in sandstone. The crystals are colourless, but many appear to be dull orange in colour, due to a dusty coating of possibly an iron mineral. The specimen is from Tan y Foel quarry at Cefn Coch in Powys. The quarry is in the Penstrowed grits formation of Mid Silurian age.