Quartz and feldspar are the commonest minerals in the Earth’s crust. Quartz is a member of the group of minerals called the silicates. It frequently forms hexagonal, elongated crystals. Clusters of quartz crystals are often found inside rocks, in circular cavities known as geodes. Geodes usually measure a few centimetres or decimetres in diameter but miners and quarry workers have occasionally come across geodes measuring more than a metre in diameter.
The most commonly occurring form of quartz crystal is a hexagonal prism ending in a six-sided pyramid. Frequently, however, the crystals have grown together in such a dense cluster that the result is a solid mass of quartz from which only part of the attractive crystal shape emerges. Quartz crystals occur in a variety of colours. They go by different names. Colourless quartz is called rock crystal. The purple crystals are called amethyst, the yellow are sold as citrine and the brownish black ones are referred to as smoky quartz.