This is a rather unusual specimen that has two major components, silica SiO2 and calcite CaCO3 It obviously started life as a silica geode, but about 30% of the centre has been invaded by a growth of massive calcite. For the main, the inner cavity is lined with miniscule quartz crystals, with a number of "stalks " composed of almost microscopic druzy quartz crystals. There are quite a number of very small doubly terminated quartz crystals, which would put them in the category of "Herkimer" diamonds. There are also quite a number of very small calcite rhombs. In the small inset picture a small rhombohedral calcite crystal is mounted on one of those Herkimer- type crystals. The specimen is from Dulcote hill in the Mendip Hills. from workings that closed in the mid 1970s.