Author name: Jens Götze

Quartz is the most popular and widely used gemstone of the SiO₂ family with its phenomenal varieties and spectacular colors, e.g., near-colorless (rock crystal), purple (amethyst), yellow (citrine), smoky (smoky quartz), and pink. The colors of these varieties are mostly caused by trace contents of foreign ions, which in turn are activated by treatment with high-energy radiation and/or temperature treatment (so-called color centers). Cryptocrystalline SiO₂ varieties such as agate, chrysoprase, or jasper mostly represent mixtures of SiO₂ minerals and other phases, while the play of colors in amorphous precious opal is caused by light diffraction and interference. Despite their different origins and properties, all these SiO₂ varieties have played a dominant role as gemstones and cut stone since antiquity.

Despite their simple chemical composition, silica (silicon dioxide, SiO₂) minerals comprise a complex system with diverse polymorphs of distinct crystal structures and different stabilities and properties. Quartz is arguably the most important mineral throughout human history, from early survival tools to critical materials for modern advanced technology. In addition, quartz and other silica polymorphs and varieties are excellent archivers and important tools for deciphering geological, environmental, and planetary processes and histories.