Specimens of calcite from Taffs Well, Wales, UK. Calcite is one of the polymorphs of calcium carbonate, the others being aragonite and vaterite. Calcite is one of the index minerals in the Mohs scale at number 3, as it is a fairly soft mineral, able to be scratched by your finger nail. As expected of this group of minerals, it reacts with hydrochloric acid or vinegar to release bubbles of CO2, this causes a reactions which results in bubbling and fizzing on the surface of the specimen. These samples of calcite have formed with a rhombohedral crystal habit and vary in colour from a semi-translucent clear crystal to opaque brown crystals. Specimens are supplied in a white card tray or perspex magnifier box with an information label, these are ideal samples for British mineral collectors.