This presentation explains how scientists label the facets of crystals using "Miller indices."
Crystals are known for their "symmetries." Meaning that they have axes around which they can be rotated to realign facets with one another.
The simplest symmetry is "cubic symmetry." Cubic crystals have three perpendicular axes (generally labeled x, y, z). Their facets are realigned by rotations of 90, 180, or 270 degrees around these axes. Diamond and most semiconductor crystals have cubic symmetry (e.g., silicon and gallium arsenide).