The Difference Between Quartz and Crystals

Quartz and crystals are often confused as being the same thing, but there are some key differences between the two. While quartz is a crystal, not all crystals are quartz.

Quartz is the second most abundant mineral on Earth after feldspar. It’s found in many different types of rock formations including sandstone, shale, granite and marble. Quartz is silicon dioxide (SiO2) and is classified as a tectosilicate mineral along with minerals like feldspars, micas and garnets. The crystal structure of quartz gives it distinctive physical properties including hardness, cleavage and how it breaks.

Pure quartz is colorless and transparent or translucent. Impurities in the mineral give quartz various colors including purple (amethyst), yellow (citrine), pink (rose quartz) and brown to black (smoky quartz). There are also several varieties of quartz like rock crystal and agate. Quartz has a vitreous luster, a Mohs hardness of 7 meaning it scratches glass, and no cleavage which means it fractures conchoidally producing curved surfaces.

In contrast, the term crystal refers more broadly to any solid material whose atoms, molecules or ions are arranged in an ordered pattern extending in all three spatial dimensions. This internal crystal structure gives the material a symmetrical physical shape, defined by flat faces and sharp angles. Not all crystals are minerals since organic compounds, metals and alloys can form crystals as well.

Crystals are identified by their shape, also known as their crystal system. There are seven crystal systems defined by the lengths of the crystal axes and the angles between them: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic. Quartz belongs to the trigonal crystal system. Its crystals exhibit triangular prisms terminated with a 6-sided pyramid at each end.

While crystal is a broad term, in geology it often refers to crystalline minerals like quartz, feldspar, olivine and calcite. These minerals crystalize slowly from molten rock, volcanic processes and water solutions. The size and shape of the crystals depends on the chemistry, temperature and time given for growth. Smaller crystals indicate rapid cooling while larger crystals suggest slower cooling.

In summary, quartz refers to a specific silicon dioxide mineral that crystallizes in the trigonal system. Meanwhile crystal is a general term for any solid material with a repetitive atomic structure and macroscopic geometric shape. All quartz is a crystal because of its internal order and external form. However most other crystals like salt, diamonds and snowflakes have a different chemical composition and crystal system than quartz. Knowing the properties of quartz can help distinguish it from other common crystals.