Calcite and quartz. Image 1 of 2. Cut and polished crystals of the minerals calcite (or calcspar, colourless) and quartz (white block). Calcite is composed of calcium carbonate. It is the main constituent of limestone, chalk and marble. It can also form stalactites or be deposited as large crystals by slowly-cooling lava. Quartz is composed of crystalline silica (silicon dioxide). It is very common and extremely hard. It is a component of nearly all rock types and is found in almost every geological environment. Photographed under white light. For the same specimens under ultraviolet light, see E425/695.
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