The 7 Crystal Systems Explained
Why fluorite forms cubes, quartz forms six-sided points, and no crystal shape is arbitrary.
Every crystalline mineral belongs to one of seven crystal systems, cubic, tetragonal, orthorhombic, hexagonal, trigonal, monoclinic and triclinic, based purely on the internal symmetry of its atomic structure. This determines the geometric shape a mineral can grow into, even before it's cut or polished.
What Is a Crystal System?
A crystal system is a category defined by the symmetry of a mineral's internal atomic lattice, specifically, the relative lengths of its crystallographic axes and the angles between them. There are seven, and every crystalline mineral on Earth belongs to exactly one.
It's worth being precise here: a crystal system describes internal structure, not outward appearance. The external shape a specimen ends up with, its habit, is influenced by the conditions it grew under, and can vary considerably even within a single crystal system.
The Cubic (Isometric) System
The highest symmetry of the seven. Three axes, all equal in length, meeting at right angles to one another. Minerals in this system tend toward highly symmetrical forms: cubes, octahedrons, and other shapes built from that same three-way equal symmetry.
Fluorite is one of the clearest everyday examples: it commonly forms in cubes, and when it breaks, it tends to break along flat cubic planes.
The Tetragonal System
Closely related to cubic, but with a twist: two of its three axes are equal in length, while the third is either longer or shorter, still meeting at right angles. The result is a structure that looks cube-like but is stretched or compressed along one direction.
The Orthorhombic System
Three axes, all of different lengths, still meeting at right angles to each other. This produces shapes that are often prismatic or blocky, but noticeably less symmetrical than cubic or tetragonal forms.
The Hexagonal System
Defined by six-sided symmetry: three equal axes at 60 degrees to each other in one plane, plus a fourth axis of a different length running at right angles to the other three. This is the system responsible for the six-sided prism shape most people picture when they imagine a crystal point.
The Trigonal System
Shares its reference axes with hexagonal and is closely related to it, closely enough that some classifications combine the two into a single system, which is why you'll occasionally see crystal systems listed as six rather than seven. Where hexagonal produces six-sided symmetry, trigonal produces three-sided symmetry.
Quartz, and by extension amethyst and citrine, belongs to the trigonal system. So does tourmaline.
The Monoclinic System
Three axes of unequal length, where two meet at right angles and the third is tilted, inclined at an angle to the others. This lower symmetry produces less regular, often flattened or elongated forms.
The Triclinic System
The lowest symmetry of all seven systems. All three axes are of different lengths, and all three angles between them are different too; none of them are right angles. A useful comparison: a triclinic crystal is a bit like a packet of envelopes pushed askew in two directions at once.
Kyanite is a well-known triclinic mineral, which is also part of why kyanite's hardness varies noticeably depending on which direction you test it from.
Crystal Systems, Answered
What determines a crystal system?
The symmetry of a mineral's internal atomic lattice, the lengths of its crystallographic axes and the angles between them.
What is the difference between crystal system and crystal habit?
Crystal system is a mineral's fixed internal symmetry. Crystal habit is the shape a specific specimen actually grows into, which can vary with conditions.
Are trigonal and hexagonal crystal systems the same?
They're closely related and share reference axes, which is why some classifications combine them into six systems. Most modern mineralogy treats them as distinct.
Which crystal system has the highest symmetry?
Cubic, with three equal axes at right angles.
Which crystal system has the lowest symmetry?
Triclinic, with three unequal axes at three unequal angles and no symmetry axis at all.