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Crystal forms Common form indices Form description Minerals that commonly exhibit crystal form Hexagonal dipyramid 12 triangular faces inclined to c‐axis; top six separated from bottom six by mirror plane Apatite, zincite Hexagonal prism Six rectangular faces parallel to c‐axis Apatite, beryl, quartz, nepheline, corundum, tourmaline Basal pinacoid {0001} Pair of faces perpendicular to c‐axis Apatite, beryl, corundum

ssss1 Common crystal forms in the trigonal system: (a) trigonal dipyramid, (b) rhombohedron, (c) scalenohedron.

Common crystal forms in the orthorhombic system include four‐sided rhombic prisms and pyramids, and eight‐sided dipyramids. Pinacoids are the dominant form and pedions are also common. Common crystal forms and associated minerals in the orthorhombic crystal system are indicated in ssss1 and ssss1.

Because of its lower symmetry, the only crystal forms in the monoclinic crystal system are four‐sided prisms, two‐sided domes, sphenoids and pinacoids, and pedions. More complex forms cannot exist in systems with low symmetry in which some crystallographic axes do not all intersect at right angles. Common crystal forms and associated minerals in the monoclinic crystal system are depicted in ssss1 and ssss1.