This term applies to transparent glass that, because of its high quality, looks like natural rock crystal. Particularly pure glass with a quantity of lead oxide over 24% is part of this category. It requires high purity primary materials and is characterised by great shininess (high index of refraction). It is used in the artistic sector, table uses and for the production of screens to protect from ionizing radiation. The need to substitute lead oxide for ecological reasons, has led to the development in recent years of glass with similar optical properties, containing potassium, barium, zinc and zirconium. The term crystal refers also to calcic-sodium-silicone glass (Venetian crystal, invented in the 15th century) and calcic-potasics (Bohemian glass).