Quartz, SiO2. Bryn y Rafr mine, west Wales. The two smaller specimens are skeletal in appearance, because the mineral which once occupied the cavities has weathered out. There are remnants of marcasite in a few places, and in the upper specimen there are numerous small cavities where impressions of the weathered mineral are well defined. The larger specimen is an example of crystal growth outwards from the two walls of a crevice or vein and meeting in the centre.