Nearly every gemstone is a crystal, which means its atoms sit in a pattern that repeats in three dimensions. Mineralogists sort those patterns into seven crystal systems by the symmetry of the pattern and the lengths and angles of a set of imaginary reference axes. The system a gem belongs to decides how light travels through it, how it splits along cleavage planes, and often what shape its rough crystals take.
Atoms, unit cells and axes
The smallest box of atoms that repeats to build a whole crystal is called the unit cell. Stack identical cells side by side in every direction and you get a crystal lattice. Because the cell repeats so exactly, the natural faces of a well-formed crystal meet at fixed angles. The Danish scientist Nicolas Steno described this constancy of angles in quartz in 1669, long before anyone could study atoms directly.
To describe the shape of the cell, mineralogists use crystallographic axes: three imaginary lines through the center of the crystal, or four in the hexagonal system. Two questions separate the systems. Are the axes the same length, and do they meet at right angles? The amount of symmetry follows from the answers. The seven systems split further into 32 crystal classes based on their exact symmetry, but gemologists work almost entirely with the seven systems.
The seven crystal systems
| System | Axes | Gem examples |
|---|---|---|
| Cubic (isometric) | Three equal axes, all at 90 degrees | Diamond, spinel, garnet, fluorite, pyrite |
| Tetragonal | Two equal axes and one of a different length, all at 90 degrees | Zircon, rutile, scapolite, idocrase |
| Hexagonal | Three equal axes at 120 degrees in one plane, plus a fourth axis of different length at right angles to them | Beryl (emerald, aquamarine, morganite), apatite |
| Trigonal | The same axes as hexagonal, with threefold instead of sixfold symmetry | Corundum (ruby, sapphire), quartz, tourmaline, calcite |
| Orthorhombic | Three unequal axes, all at 90 degrees | Topaz, peridot, chrysoberyl, zoisite (tanzanite), iolite |
| Monoclinic | Three unequal axes, two at 90 degrees and one inclined | Spodumene (kunzite), orthoclase (moonstone), jadeite, diopside, malachite, titanite |
| Triclinic | Three unequal axes, none at 90 degrees | Kyanite, turquoise, labradorite, microcline (amazonite), rhodonite |
Some older books list six systems and treat trigonal as a division of hexagonal. Both approaches describe the same crystals.
Cubic
The cubic system has the most symmetry. Its crystals tend to be equant, about as wide as they are tall, and grow as cubes, octahedra or twelve-sided dodecahedra. Diamond usually forms octahedra, spinel forms sharp octahedra, and garnet forms rounded dodecahedra and 24-sided trapezohedra.
Tetragonal
Tetragonal crystals are often square in cross section and longer in one direction. Zircon grows as four-sided prisms capped by pyramids. Rutile, also tetragonal, forms the golden needles seen in rutilated quartz.
Hexagonal and trigonal
Beryl is the classic hexagonal gem. Emerald and aquamarine crystals are six-sided prisms that often end in a flat face. Trigonal crystals can look six-sided too, but their symmetry repeats every 120 degrees instead of every 60. Corundum, the mineral behind ruby and sapphire, forms barrel shapes or flat plates. Quartz forms six-sided prisms topped by pyramid-like points. Tourmaline prisms have a rounded triangular cross section and fine grooves running along their length.
Orthorhombic
Orthorhombic gems include topaz, with its striated prisms and perfect cleavage across the length, and peridot. Chrysoberyl is orthorhombic as well, and its crystals often twin into shapes that look six-sided, a habit well known in alexandrite. Zoisite, the mineral behind tanzanite, also falls here.
Monoclinic
Monoclinic gems have one tilted axis. Kunzite (spodumene) and orthoclase moonstone belong to this system, along with jadeite, diopside and titanite.
Triclinic
Triclinic crystals have the least symmetry of all. Kyanite is a good example: its hardness changes depending on the direction you scratch it, a direct result of its lopsided structure. Labradorite and turquoise are triclinic too, though turquoise almost never shows visible crystals.
Amorphous and aggregate gem materials
Some gem materials have no ordered atomic lattice at all. Mineralogists call them amorphous, and natural amorphous solids are known as mineraloids. Opal is the best-known gem example. It is built from tiny spheres of silica holding water. In precious opal those spheres are stacked in a regular array that splits light into colors, but the silica itself is amorphous or very poorly crystalline. Obsidian, natural glass such as moldavite, man-made glass and amber are also amorphous.
Amorphous materials have no crystal system, never grow natural crystal faces, and usually break with smooth, shell-shaped fractures. They are singly refractive, though internal strain can cause odd patterns in a polariscope.
Other gems are aggregates: masses of many tiny crystals grown together. Chalcedony is made of microscopic quartz crystals. Jade is a mat of interlocking jadeite or nephrite crystals, and that felted texture explains its toughness. Lapis lazuli is a rock made of several minerals. Each tiny grain has its own crystal system, but because the grains point in every direction, the mass stays bright in a polariscope as it rotates, a reaction gemologists call an aggregate reaction.
Why crystal systems matter for optics
Light behaves differently depending on the crystal system. In cubic and amorphous materials, light travels at the same speed in every direction. These materials are isotropic and singly refractive, so a refractometer shows one reading.
In the other six systems, light entering the stone splits into two rays that travel at different speeds. This is double refraction, and the gap between the two readings is the birefringence. Tetragonal, hexagonal and trigonal gems are uniaxial: they have one direction, the optic axis, along which light does not split, and they show two main refractive index values. Orthorhombic, monoclinic and triclinic gems are biaxial, with two optic axes and three main refractive indices.
Crystal system also limits pleochroism, the way some colored gems show different colors from different directions. Uniaxial gems can show at most two colors. Biaxial gems can show three, which is why tanzanite can flash blue, violet and a third tone depending on how it is turned. Isotropic gems show only one color in every direction.
These rules help in identification. A singly refractive red stone could be spinel or garnet, while a doubly refractive one could be ruby. They also guide cutters. Ruby usually looks its best red when the table is cut at right angles to the optic axis. Dark tourmaline looks almost black down its length, so cutters lay the table parallel to the prism instead. Cleavage follows the structure too: cubic diamond cleaves in four directions parallel to its octahedron faces, while orthorhombic topaz cleaves in one direction. The page on gemstone cleavage covers how cutters and setters deal with this.
Polymorphs: same chemistry, different system
The same chemical recipe can crystallize in more than one system, depending on the temperature and pressure where it grows. These different structures are called polymorphs. Carbon forms cubic diamond under high pressure deep in the mantle and hexagonal graphite under ordinary conditions. Calcium carbonate forms trigonal calcite and orthorhombic aragonite, and aragonite is the mineral that builds the nacre of pearls. Aluminum silicate forms three minerals: triclinic kyanite and orthorhombic andalusite and sillimanite. Geologists use these three to read the pressure and temperature a rock went through.
Frequently asked questions
How many crystal systems are there?
There are seven: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic. Some books list six by grouping trigonal with hexagonal.
Which crystal system does diamond belong to?
Diamond is cubic. That is why it is singly refractive and why it cleaves along four directions parallel to the faces of an octahedron.
Does opal have a crystal system?
No. Opal is an amorphous mineraloid, so it has no crystal lattice in the usual sense, though precious opal has an orderly stacking of silica spheres that produces play-of-color.
Can you tell a crystal system by looking at a cut gem?
Not by shape, because cutting removes the natural faces. Gemologists infer the system from optical tests such as single or double refraction, uniaxial or biaxial readings, and pleochroism.