Rock crystal

Rock crystal (in Turkish “necef taşı” or “dağ kristali”) is the everyday name for colourless, transparent quartz. Chemically it is silicon dioxide (SiO₂); in nature it usually grows as six-sided prisms with pointed tips and is hard enough to scratch glass. In everyday language the word “crystal” is used for three different things: clear quartz (rock crystal), any mineral specimen sold in crystal shops, and the sparkling lead glass used in chandeliers and drinking glasses. In science, a crystal is a solid whose atoms are arranged in a regular, repeating pattern: salt, sugar, snowflakes and quartz are crystals, whereas glass, with its disordered atoms, is not.

Ornaments and jewellery: Rock crystal is worked into pendants, beads, spheres, bowls and figurines; in the past it was also carved into cups, ewers and seals. Collecting: Clear clusters with intact points are among mineral collectors’ favourite specimens. Technology: Quartz’s piezoelectricity is used to keep time in watches and electronic circuits; synthetic quartz crystals grown for the purpose are preferred today. Optics: Because quartz lets ultraviolet light through, it is used in some optical components.

Natural rock crystal usually ends in a six-sided prism with a pointed tip; fine horizontal lines can be seen across the prism faces. It is hard (Mohs 7): a steel knife cannot scratch it, but it scratches glass. Lead crystal glass is much softer and can be scratched with a knife. It feels cold in the hand and stays cold for a long time; inside there are often fine, veil-like cracks and small inclusions. Glass spheres show round air bubbles and swirl marks instead. Chandelier drops, crystal glasses and decanters, and “crystal” ornaments and stones are mostly glass containing additives such as lead or barium. Under European Union rules, glass containing at least 24 percent lead oxide may be called “lead crystal”.

On the “street to diamond” value ladder it sits on step 5 of nine: Ornamental stone. Rock crystal is a common stone; its value depends on size, clarity, the shape of the crystal and its completeness (undamaged tips). Collectors prize large, clear clusters with intact points; in ornaments, spheres and carvings made from large, inclusion-free pieces are worth more. Crystals with interesting inclusions such as needles of rutile are also sought after. The value of lead crystal glass comes not from any stone but from glassmaking skill and cutting.

Since the Middle Ages people have believed that the future can be seen by gazing into spheres of rock crystal (crystal gazing). Today, stone sellers and crystal therapy sources claim that clear quartz cleanses “negative energy” from homes and people, clears the mind and sharpens focus. The same sources say crystals must be “purified” by leaving them in moonlight or in salt water. There is no scientific evidence that rock crystal cleanses energy, strengthens the mind or reveals the future. Piezoelectricity, a real physical property of quartz, produces only tiny amounts of electricity when a crystal is squeezed and has no effect on the human body. Soaking crystals in salt water can damage some soft or water-sensitive minerals. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.

Natural rock crystal is harmless. Lead crystal, however, can release lead into drinks; the amount rises sharply when acidic drinks such as wine, liqueurs and fruit juice are kept in decanters for long periods. Do not store drinks in lead crystal or give it to babies and children. Remember too that a crystal sphere can focus sunlight like a lens and start a fire.