|4.67 mm complex purple Fluorite crystal|
Crystals are the outer form of a mineral's inner order. Nearly every piece of rock of Earth is made of minerals, each of which is defined as a natural substance with a precise recipe and arrangement of atoms. It's important to realize that every mineral, whether it looks like a crystal or not, is crystallized inside. The outer shape of a crystal has a complicated relationship to that internal structure. The inside is the important part—how the various atoms of the mineral fit together.
Every mineral can be modeled as an assembly of unit cells. The unit cell is the smallest set of atoms that would have all the chemistry of the mineral, and it corresponds to the mineral's formula. So halite, NaCl, has a unit cell consisting of a sodium atom and a chlorine atom. Complicated minerals like the micas or the clays have larger ones. But just as a pile of fruit in a grocer's display is partly determined by the general shape of the particular fruit, the pattern made by the interlocking unit cells in a mineral is an important clue to the outward shape taken by that mineral's crystals.
The internal structure of a crystal is determined using X-rays, which are light of an extremely small wavelength. The wavelength of X-rays is small enough that they can reflect off of rows and planes of atoms inside a crystal just as easily as visible light bounces off of mirrors. Think of the ordered atoms—the molecular lattice—of a crystal like the arrangement of trees in an orchard as you drive past. The lanes that you see running through those trees, some wide and some narrow, are like the features that make X-rays reflect as they shine through the mineral lattice.
The lattice of an orchard is simple. The lattices of minerals range from simple to very complex, and they are part of what governs how crystals grow. In most rocks, minerals can only grow until they meet the grains that surround them, so while they are crystalline inside, they can't express the external form that we think of as crystals. Crystals need a space, a void, around them before they can form the flat faces we prize. But when they have that space, whether it's because they're the first minerals to form in a magma or because they're growing in an opening of some sort, they acquire shapes that crystallographers have spent centuries studying. The outer shapes of crystals reflect not just their internal structure, but the details of the environment around them as they grew.
The simplest rule of crystals was first described in 1667 by Nicolaus Steno: Whatever shape they have, crystals of a mineral always have the same angles between the faces. Those are direct clues to the internal structure—the real crystallinity—of a mineral, but they are clues that we couldn't fully understand until physics and chemistry advanced in the 20th century. It takes some work to gain a better understanding of crystals as geologists understand them; this article is just bringing you to the threshold of the subject.