Quartz, SiO2. Pencurig mine, Dylife, Central Wales Orefield. These specimens were taken from the wall of an adit. I have labelled them as unclassified because of their peculiar nature. The quartz appears to be fractured and friable, and can be easily broken by hand. Friable quartz can be found as a sedimentary deposit, but this quartz is from a hydrothermal source.