When minerals form in environments where they can grow without interference from neighboring grains, they commonly develop into regular geometric shapes, called crystals, bounded by smooth crystal faces. The crystal form for a given mineral is governed by the mineral's internal structure, and may be distinctive enough to help identify the mineral. For example, quartz forms elongated, six-sided prisms capped with pyramid-like faces; galena and halite occur as cubes; and garnets develop 12- or 24-sided equidimensional forms. Interference from other mineral grains during growth may prevent formation of well-formed crystals. The result is shapeless masses or specimens that developed only a few smooth crystal faces. This type of specimen is much more common than well-formed crystals.