To most people, a crystal is a sparkling gem-like solid with well-formed faces and a geometric shape. For many scientists, including all mineralogists, crystal and crystalline also refer to any solid compound having an ordered, repetitive, atomic structure, which may or may not result in crystal faces and a gemmy appearance.
We use the term “crystal” in both ways. When a mineralogist refers to a garnet crystal, the reference may be to a dodecahedron, a twelve-sided crystal with diamond-shaped faces like the drawing seen in Figure 3.5 and the real garnet next to it in Figure 3.6. We describe crystals, such as the well-formed garnet crystal with well-developed faces, as being euhedral.
On the other hand, petrologists and mineralogists may refer to crystals of garnet in a rock. The crystals may not have any smooth faces at all. The garnet seen in the photo here, which was picked out of garnet gneiss (a metamorphic rock), lacks crystal faces entirely. If no faces are visible, as in this photograph, the crystal is anhedral. Those crystals that fall between euhedral and anhedral are called subhedral.
Mineral crystals always have an ordered arrangement of atoms within them, but the crystals may not be geometrically shaped or smooth on the outside. All garnet crystals, for example, have the same highly ordered arrangement of atoms shown in this ball and stick drawing (Figure 3.8), but only some garnet crystals have visible crystal faces. In fact, most natural garnet is anhedral or, perhaps, subhedral. So, the garnet in the photo above is typical.
With just a few exceptions, all minerals are crystalline, but perfectly formed crystals with flat faces are relatively rare. Nonetheless, because crystal shape reflects the crystal’s atomic arrangement, when faces on a mineral are fully or partially developed, crystal shape can be a powerful identification tool. When no faces are visible, we must rely on other properties to identify a mineral.