Zircon

Zircon is a common silicate mineral, zirconium silicate (ZrSiO4), and the main source of the metal zirconium. It turns up as small grains in igneous, metamorphic and sedimentary rocks, usually well under a millimetre across in granite, though pegmatites and carbonatites can grow crystals several centimetres long. Fairly hard (7.5 on the Mohs scale), dense and chemically stubborn, it outlasts weathering and ends up in most sands. Natural stones run from colourless through yellow, red and brown to blue and green; heating can turn ordinary brown zircon colourless or blue, and blue is the most prized. Its high refractive index has led to its use in place of diamond, though its strong double refraction usually gives it away. Many zircons hold traces of uranium and thorium, so Wikipedia lists the mineral as radioactive; that same content lets geologists date single grains, and zircons from Western Australia's Jack Hills, about 4.4 billion years old, are the oldest minerals yet dated on Earth. Cunningham calls it a confusing stone, found in many colours, some of them artificial, and under many names. He gives it to the Sun and to fire, pairs it with gold, and counts love, protection, healing, peace, beauty, sexual energy and guarding against theft among its powers.