The seven crystal systems

A mineral's atomic identity card

A crystal is an arrangement of atoms repeated identically in all three dimensions of space. That arrangement obeys strict geometric rules, and there are in all only seven crystal systems possible in nature. That is very few — and it is what makes crystallography so elegant.

The seven families

The cubic (or isometric) system is the most symmetrical: three equal, perpendicular axes. Diamond, garnet, fluorite and pyrite belong to it — and pyrite's natural cubes are so perfect they look machine-cut. A remarkable property: cubic minerals never show pleochroism, which makes it an identification test.

The tetragonal system (zircon, rutile) has two equal axes and a third that differs. The hexagonal (emerald, aquamarine, apatite) yields those recognisable six-sided prisms. The trigonal — often grouped with hexagonal — gathers quartz, calcite, ruby, sapphire and tourmaline: it is the most represented system in gemmology.

The orthorhombic (topaz, peridot, tanzanite) has three unequal but perpendicular axes. The monoclinic (jade, malachite, azurite, selenite) and the triclinic (labradorite, amazonite, turquoise) are the least symmetrical, with oblique axes.

And the stones with no structure?

Some precious materials have no crystal structure at all: these are the amorphous bodies. Opal is made of stacked silica microspheres, but with no repeating atomic lattice. Obsidian is a volcanic glass cooled too fast to crystallise. Amber is an organic resin. These materials are therefore not, strictly speaking, minerals — and they have neither cleavage, nor pleochroism, nor a crystal form of their own.

What it is actually for

The crystal system determines very practical properties: cleavage (fluorite breaks along clean planes, quartz never does), the presence or absence of pleochroism, and how the lapidary must orient a stone to cut it without shattering it. A jeweller who ignores a topaz's cleavage is heading for disaster.