Calcite, calcium carbonate, CaCO3. Nantiago mine, Central Wales Orefield. This particular specimen appears to be composed of myriads of very small rhombohedral crystals, many of which are arranged in regimented rows or columns. There are a few small vughs where the construction of the crystals can be better observed, albeit with the aid of a microscope. There are also several small well formed cubes of pyrite inside the vughs.