Thematic Articles

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.

The use of microcrystalline silica has been documented among early hominins, with significant diversification over the last ~3 million years, from early toolmaking to applications in personal adornment and symbolic contexts. From the Oldowan onwards, both microcrystalline quartz and amorphous silica are consistently observed in the archaeological record, and they shaped aspects of human technological and behavioral evolution. This article offers insights into how mechanical, environmental, and cultural factors guided the selection of silica materials and their transformation to purposeful objects—from Pliocene hominins through to the Middle Ages. The crystal chemistry that imbues microcrystalline quartz varieties with their unique properties is reviewed, and the formation pathways for biogenic and abiogenic microcrystalline and amorphous silica are considered.

Humankind’s use of silica raw materials began at least 1.85 million years ago with production of flint tools and spans to today’s more than 1000 industrial applications, all relying on its chemistry and specific physical properties. Among the key achievements are the discovery of the piezoelectric effect of quartz for timekeeping, the critical importance of silicon-based semiconductor technologies for the digitalization of our society, and the production of renewable energy through photovoltaics. One of the greatest challenges regarding raw material supply and security lies in identifying new quartz deposits of sufficient purity, size, and properties to meet increasing demands. Although quartz is mined from a variety of deposits, economically viable sources remain difficult to locate for reasons that we explain.

Quartz is a key archive of Earth surface processes, recording erosion, burial, transport, and landscape evolution across a wide range of timescales. Recent advances in quartz-based analytical techniques have expanded the potential for integrative geochemical and geochronological approaches. Fourier-transform infrared spectroscopy (FTIR) enables detailed characterization of quartz provenance and geological history, while terrestrial cosmogenic nuclides (TCNs) constrain surface exposure, burial histories, and denudation rates. Optically stimulated luminescence (OSL) provides robust age control for fine-grained sediment transport and deposition, and ongoing developments in electron spin resonance (ESR) extend chronological constraints into previously inaccessible time ranges. Understanding the principles and limitations of each method is essential for their effective individual and combined applications in reconstructing Earth surface processes.

Quartz and other silica minerals are amongst the most important constituents of the Earth’s crust, both as rock-forming mineral components and as tools for understanding Earth processes. Quartz and other forms of silica are essential parts of both the geological and biochemical cycles with the potential to help us understand the processes of our planet across disciplines. This article is a narrative of quartz, starting with its crystallization from fractionated, silica-rich magmas, transport, and deposition of silica by hydro- thermal fluids, as well as physical, chemical, and biogenic processes resulting in silica weathering, transport, accumulation, and the formation of sedimentary silica deposits. Extraterrestrial quartz and silica occurrences are also discussed.

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.