Specimens of rhombohedral calcite crystals from Machen Quarry, South Glamorgan, Wales. 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 as rhombohedral crystals in various growth stages upon a matrix of dolomite or calcite. The crystals vary in colour from a translucent beige to an opaque white and may even show a small amount of stepping. Specimens are supplied in a white card tray with an information label and are a must have for any British mineral collector.