Believed to be a permanent form of frozen water by some early cultures due to its clearness and coolness to the touch, quartz is a common crystal. It makes up around 20 percent of the Earth’s crust and around 12 percent of the planet’s land surface. This reflects the fact that quartz is the crystalline version of silicon dioxide (SiO2). The Earth’s two most common elements are silicon and oxygen, which combine to form SiO2.
At the atomic level, quartz is formed when silicon and oxygen join together as three-sided pyramids or tetrahedrons. These stack together to create crystals, with billions of tetrahedrons required to form even a tiny crystal. This structure is not unique to quartz and there are other structures in other crystals (known as habits). What is unique about quartz is SiO2 composition, plus a crystalline structure.
There are two basic types of quartz: crystalline and cryptocrystalline. Growing as large, visible crystals, crystalline quartz is cut by lapidaries into gemstones such as citrine, amethyst, and rock crystal. (Lapidaries, those who cut and polish gems, do not manipulate the color of quartz, but rather showcase sought-after attributes in gemstone types). The crystals may be present in many sizes and forms, from tiny crystal frostings that sparkle distinctively on rock surfaces to well-formed crystals that weigh in tons.
When in cryptocrystalline form, quartz contains microcrystals, which can only be seen with a microscope. This type is cut by lapidaries into forms such as jasper, agate, and other chalcedony.
Nearly pure as a chemical compound, quartz has constant physical properties and a 7.0 Mohs hardness, which makes it ideal as a gemstone. Its abundance and hard crystal structure also make it the Earth’s most common weathering-resistant mineral, and it’s the primary component of beach sand. Its sturdiness makes it an important material in glass, including the Pyrex heat-resistant type, ceramics, and stoves that contain halogen heating elements.
In igneous rocks, quartz crystallizes from magma, forming as the molten material cools. Similar to how water transforms into ice, silicon dioxide crystalizes as the temperature is gradually reduced. This type of quartz is often found in granite and other types of igneous rock.
By contrast, hydrothermal quartz precipitates around geothermal vents in silica-rich water. First, a combination of pressure and high temperature causes silicon to dissolve into water (as salt will at much lower temperatures and pressures). When the pressure or temperature falls, the solution becomes saturated. This means that the maximum amount of solute for the solution at the specific pressure or temperature has been reached. The excess silicon dioxide forms as quartz crystals.
The sought-after large and long quartz crystals, often clustered in groups, are formed when circulating fluids at depth dissolve the silica from existing quartz rock. This eventually precipitates out into quartz as the silica-rich fluids flow into and are deposited within cracks and cavities underground, forming geodes and veins.
Sedimentary quartz also exists, which involves rocks on the Earth’s surface weathering and eroding, such that silica releases into water. This precipitates out into quartz grains, which inhabit sandstone and other types of sedimentary rock. Finally, metamorphic quartz is formed when igneous or sedimentary rocks containing quartz are subject to intense heat and pressure, as when continental plates collide. This causes the silica to recrystallize, forming quartz.
Quartz, then, is a common, yet often sought after, gemstone that forms in various ways, as pressure and temperature bring changes to the chemical composition of rock, water, and magma. Among the unique items at the Smoky Mountain Relic Room in Sevierville, Tennessee, is a giant amethyst geode that has been split open to reveal “butterfly wings,” with deep purple giving way to clear crystal, a result of differential sunlight exposure over the millennia.
