Crystal Systems: The Seven Blueprints Behind Every Gemstone
Why does Pyrite so often grow as a crisp cube, while Rock Crystal ends in a six-sided point? The answer lies hidden deep inside the stone. Every gemstone is built according to a fixed blueprint of atoms, and that blueprint is called the crystal system. On this page you will learn what a crystal system is, why there are seven of them rather than six, and which well-known gemstones belong to each system. After reading, you will look at the stones on your shelf with new eyes.
What Is a Crystal System?
A crystal is made up of atoms arranged in a fixed, repeating pattern: the crystal lattice. The smallest building block of that lattice is called the unit cell. Stack that cell billions of times in every direction and you get a crystal you can hold in your hand.
The shape of the unit cell determines far more than appearance alone. It decides how symmetrically a crystal grows, in which directions a stone cleaves easily and how light travels through it. Based on that symmetry, mineralogy places every crystal in one of seven crystal systems. Each mineral belongs to exactly one system, and all minerals within a system share the same geometric foundation.
Six or Seven Crystal Systems?
In mineralogy there are seven crystal systems: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal and trigonal. Some gemology books say there are six. That is because the hexagonal and trigonal systems share the same arrangement of axes and together form a single crystal family. If you only look at the axes, you count six.
The symmetry, however, is fundamentally different. A hexagonal crystal has a sixfold axis of symmetry: turn it one sixth of a rotation and it looks the same again. A trigonal crystal has a threefold axis and only looks the same after one third of a rotation. Quartz is trigonal, Beryl is hexagonal. Precisely because the Quartz family is so large and so loved, we at Kristalmeester work with seven systems.
1. The Cubic Crystal System
The cubic system (also called isometric) is the most symmetrical of all. Three axes of equal length meet at right angles, so crystals grow evenly in every direction. Typical forms are the cube, the octahedron (two pyramids joined at their bases) and the dodecahedron with twelve diamond-shaped faces, the classic shape of Garnet.
Because of this high symmetry, cubic stones refract light singly and show no change of color when you turn them. That gives them a calm, even appearance.
Well-known examples: Pyrite, Fluorite, Garnet, Spinel, Sodalite, Lapis Lazuli (the blue mineral Lazurite) and Diamond.
2. The Tetragonal Crystal System
The tetragonal system resembles the cubic one: here too, three axes meet at right angles. The difference is that one axis is longer or shorter than the other two. As a result, the crystals grow as prisms with a square cross section, often topped by a pyramid.
This system occurs less frequently in nature than the others, but it does produce a few striking minerals.
Well-known examples: Rutile (the golden needles in Rutilated Quartz), Apophyllite, Vesuvianite, Zircon, Cassiterite and Wulfenite.
3. The Orthorhombic Crystal System
In the orthorhombic system the three axes also meet at right angles, but now all three differ in length. The result is often elongated, rectangular or lens-shaped crystals. Think of the typical columnar shape of Topaz or the flat, tabular crystals of Barite.
Stones from this system sometimes show a beautiful play of color as you turn them. Tanzanite is the best-known example: blue from one direction, violet from another.
Well-known examples: Topaz, Peridot, Aragonite, Barite, Celestine, Iolite and Tanzanite.
4. The Monoclinic Crystal System
The monoclinic system has three unequal axes, two of which meet at right angles. The third axis is inclined. Crystals from this system therefore often have a slightly skewed, tilted shape. It is the most common crystal system in nature: well over a third of all known minerals belong to it.
Many beloved stones from this system are known mainly in polished form, because in nature they rarely occur as loose, well-formed crystals. Malachite and Jade, for example, usually grow as dense, fibrous masses.
Well-known examples: Malachite, Azurite, Kunzite, Selenite (Gypsum), Moonstone and Orthoclase, Diopside, Jade and Nephrite, Epidote, Lepidolite and Serpentine.
5. The Triclinic Crystal System
The triclinic system is the least symmetrical. All three axes differ in length and none of the angles is 90 degrees. The crystals therefore look irregular and quirky, with faces that never quite meet at right angles. It is exactly this individuality that makes them so natural and lively.
A fine example of what low symmetry does is Kyanite: the stone is noticeably softer along its length than across it. The feldspars Amazonite and Labradorite also owe their play of color and characteristic luster to this triclinic structure.
Well-known examples: Amazonite, Labradorite, Turquoise, Rhodonite and Kyanite.
6. The Hexagonal Crystal System
The hexagonal system works with four axes. Three axes of equal length lie in one plane and meet at angles of 120 degrees. The fourth axis stands perpendicular to them and is shorter or longer. This structure produces the characteristic six-sided columns with sixfold symmetry.
The Beryl family is the showpiece of this system. A well-formed Aquamarine or Emerald displays the hexagonal cross section beautifully.
Well-known examples: Aquamarine, Emerald, Morganite and Apatite.
7. The Trigonal Crystal System
The trigonal system uses the same four axes as the hexagonal one, but its symmetry is threefold rather than sixfold. Look at the tip of a Rock Crystal point and you will often see that the six faces of the termination alternate between large and small: three large and three small. That is the trigonal symmetry shining through the six-sided column. Other typical forms are the rhombohedron (a squashed cube) of Calcite and the triangular cross section of Tourmaline.
For gemstone lovers this may well be the most important system of all, because the entire Quartz family belongs to it, as does Corundum (Ruby and Sapphire). Agate, Chalcedony and Jasper are trigonal too: they consist of Quartz in microscopically small crystals, which is why you cannot see the crystal shape with the naked eye.
Well-known examples: Rock Crystal, Amethyst, Smoky Quartz, Citrine, Rose Quartz, Aventurine, Agate, Chalcedony, Calcite, Black Tourmaline, Ruby, Sapphire, Hematite and Rhodochrosite.
Amorphous Stones: Gemstones Without a Crystal System
Not every gemstone is a crystal. Amorphous materials have no ordered crystal lattice: the atoms lie jumbled together, as in glass. They often form when molten rock cools so quickly that the atoms have no time to arrange themselves neatly. Obsidian and Moldavite are examples of this. Opal consists of minute silica spheres with water in between, and Amber and Jet are of plant origin.
Strictly speaking, amorphous stones are therefore not minerals, but as gemstones they are no less loved.
Well-known examples: Obsidian, Moldavite, Opal, Amber and Jet.
The Seven Crystal Systems at a Glance
Cubic
Axes: 3 equal axes, all angles 90°
Form: cube, octahedron, dodecahedron
Examples: Pyrite, Fluorite, Garnet, Spinel
Tetragonal
Axes: 3 axes, 1 of different length, all angles 90°
Form: square prism, bipyramid
Examples: Rutile, Apophyllite, Zircon
Orthorhombic
Axes: 3 unequal axes, all angles 90°
Form: rectangular column, tablet
Examples: Topaz, Peridot, Tanzanite
Monoclinic
Axes: 3 unequal axes, 1 inclined angle
Form: skewed prisms, dense masses
Examples: Malachite, Kunzite, Selenite, Jade
Triclinic
Axes: 3 unequal axes, no 90° angles
Form: irregular blocks and plates
Examples: Amazonite, Labradorite, Turquoise
Hexagonal
Axes: 4 axes, sixfold symmetry
Form: six-sided column
Examples: Aquamarine, Emerald, Apatite
Trigonal
Axes: 4 axes, threefold symmetry
Form: six-sided column with point, rhombohedron
Examples: Rock Crystal, Amethyst, Ruby, Tourmaline
Why Is This Useful to Know?
Knowing the crystal systems makes you a better observer. You understand why an Amethyst cluster is made of points and a Malachite of curved bands. You spot imitations faster: glass is amorphous and never shows the natural crystal faces or growth lines of real Quartz. And you understand why some stones need extra care. Selenite, Kyanite and Topaz cleave easily along fixed planes in their lattice, so those are best not dropped.
Frequently Asked Questions about Crystal Systems
How many crystal systems are there?
There are seven crystal systems: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal and trigonal. Some sources count six, because they group trigonal with hexagonal. Both systems share the same axes, but their symmetry differs.
What is the difference between hexagonal and trigonal?
Both systems work with four axes, but a hexagonal crystal has a sixfold axis of symmetry and a trigonal crystal a threefold one. Beryl (Aquamarine, Emerald) is hexagonal, Quartz (Rock Crystal, Amethyst) is trigonal.
Which crystal system does Rock Crystal belong to?
Rock Crystal and all other Quartz varieties, such as Amethyst, Smoky Quartz, Citrine and Rose Quartz, belong to the trigonal crystal system. The six-sided column with a pointed termination is its typical form.
Are Opal and Obsidian crystals too?
No. Opal, Obsidian, Moldavite, Amber and Jet are amorphous: they have no ordered crystal lattice. They therefore belong to no crystal system at all, yet they are still used as gemstones.
Can you tell the crystal system from a tumbled stone?
Usually not. Tumbling and polishing remove the natural crystal faces. The inner order, however, remains fully intact: a tumbled Amethyst is just as trigonal as a rough Amethyst point.
In Conclusion
Every gemstone carries an inner order. The seven crystal systems form the silent foundation on which color, shape and growth rest. Once you learn to recognize that structure, you look at even the smallest stone with more wonder.
If you have any questions about crystal systems or about a specific stone, please do not hesitate to contact Kristalmeester. We are happy to help.