In crystallography, a crystal form is a set of crystal faces that are related to one another by the symmetry operations of a given crystal class. The concept is fundamental to gemmology: the characteristic shapes of ruby's hexagonal prisms, diamond's octahedra, and garnet's rhombic dodecahedra are all direct expressions of the forms permitted by each mineral's internal symmetry. Understanding crystal forms allows the gemmologist to identify species in the rough, to anticipate cleavage and parting directions, and to interpret the growth history recorded in a stone's external geometry.

From Symmetry to Face

Every crystal belongs to one of thirty-two crystal classes, each defined by a specific combination of symmetry elements — rotation axes, mirror planes, inversion centres, and rotoinversion axes. When a single face is placed in a general orientation within such a class and then replicated by every symmetry operation the class possesses, the complete set of equivalent faces produced constitutes a crystal form. The number of faces in a form therefore depends directly on the richness of the class's symmetry: a face in the cubic system, which has the highest symmetry, may generate as many as forty-eight equivalent faces, whereas a face in the triclinic system, which has minimal symmetry, may remain as a solitary, unrepeated plane.

Forms are conventionally denoted by enclosing the Miller indices of a representative face in braces — for example, {100} for the cube, {111} for the octahedron, {110} for the rhombic dodecahedron. These notations appear throughout gemological literature and laboratory reports when describing rough crystal morphology.

Open and Closed Forms

A critical practical distinction separates closed forms from open forms. A closed form is one whose faces, taken together, entirely enclose a volume of space; a crystal consisting solely of a closed form can therefore exist as a complete, self-contained solid. The cube, octahedron, tetrahedron, rhombic dodecahedron, and trapezohedron are all closed forms. Diamond most commonly crystallises in the octahedron {111} and, less frequently, in the cube {100} or rhombic dodecahedron {110} — all closed forms capable of producing complete, bounded crystals.

An open form, by contrast, cannot enclose space on its own and must combine with other forms to produce a finite crystal. Open forms include:

• Pedion — a single, isolated face with no symmetry-equivalent counterpart; found only in the lowest-symmetry classes.
• Pinacoid — a pair of parallel faces related by an inversion centre or a two-fold axis; common in triclinic and monoclinic minerals.
• Prism — a set of faces parallel to one principal axis, forming an open tube; the hexagonal prism {10ī0} is the dominant form of corundum (ruby and sapphire), beryl (emerald and aquamarine), and tourmaline.
• Pyramid — faces meeting at a point on the principal axis but open at the opposite end.
• Dipyramid — two pyramids joined base-to-base, enclosing space and therefore a closed form; the dihexagonal dipyramid characterises well-formed beryl crystals.
• Dome and sphenoid — two non-parallel faces related by a mirror plane or a two-fold axis respectively.

General and Special Forms

Forms are further classified by the orientation of their faces relative to the symmetry elements of the crystal class. A general form has faces that intersect all crystallographic axes at unequal, non-zero distances and are not parallel to any symmetry element. In any crystal class, the general form possesses the maximum number of faces possible for that class and is the form from which the class takes its name — hence the cubic system's general form, the hexoctahedron {hkl}, gives rise to the alternative name hexoctahedral class for the highest-symmetry cubic class.

A special form has faces that are parallel to, or intersect at special angles with, one or more symmetry elements. Because some symmetry operations leave such faces unmoved rather than generating new ones, special forms have fewer faces than the general form of the same class. The cube {100} and octahedron {111} are both special forms of the cubic system; the cube's six faces are each perpendicular to a four-fold rotation axis, and the octahedron's eight faces are each perpendicular to a three-fold axis.

Crystal Habit: Forms in Combination

The external shape actually observed on a natural or synthetic crystal — its habit — is almost always the result of several forms developed simultaneously, each growing at its own rate under the prevailing conditions of temperature, pressure, fluid chemistry, and available space. A ruby crystal, for instance, typically combines the hexagonal prism {10ī0} with the basal pinacoid {0001} and one or more rhombohedral forms {10ī1}, producing the stubby, barrel-shaped or tabular outline familiar from Mogok and Mong Hsu rough. Elongated prismatic habit, by contrast, is favoured when prism faces grow slowly relative to the terminating pyramids or rhombohedra — a condition often associated with rapid crystallisation or particular fluid compositions.

Emerald offers a well-documented example of habit variation tied to genesis. Crystals from Colombian hydrothermal veins tend to be long, slender hexagonal prisms with poorly developed terminations, reflecting rapid axial growth along the c-axis. Schist-hosted emeralds from Zambia and Zimbabwe more frequently display shorter, stouter prisms with prominent dipyramidal terminations, consistent with slower, more isotropic growth environments. These habit differences carry practical consequences: the Colombian habit yields rough better suited to step-cut rectangles, while the more equidimensional Zambian habit accommodates cushion and oval outlines with less weight loss.

Gemmological Significance

Crystal form is not merely an academic classification; it has direct diagnostic and practical value across several areas of gemmological practice.

• Species identification in the rough: The combination of forms present, together with interfacial angles measured by contact goniometer or estimated visually, often permits identification before any optical or chemical testing. Spinel's octahedral habit, topaz's characteristic orthorhombic prism with basal cleavage, and tourmaline's trigonal prism with striated faces are each diagnostic.
• Cleavage and parting prediction: Cleavage planes are always parallel to actual or possible crystal faces — that is, to planes belonging to a form of the crystal's class. Knowing the forms present allows the lapidary to anticipate where a stone will cleave and to orient the rough accordingly. Topaz's perfect basal cleavage {001} is parallel to the pinacoid, and this single fact governs every decision about how topaz rough is oriented for cutting.
• Inclusion interpretation: Negative crystals — fluid- or gas-filled cavities that preserve the form of a missing crystal — adopt the same forms as positive crystals of the host mineral. The three-phase inclusions in Colombian emerald, for example, often occur within negative crystals bounded by prism and rhombohedron faces, confirming crystallographic control over cavity morphology.
• Synthetic and treated stone detection: Flux-grown synthetic spinels and garnets sometimes display unusual combinations of forms, or forms developed in proportions not observed in natural crystals from any known locality, providing one line of evidence for synthetic origin.

The Thirty-Two Crystal Classes and Their Forms

Each of the thirty-two crystal classes has its own repertoire of permitted forms, ranging from the single pedion of the pedial class (class 1, triclinic) to the forty-seven distinct forms of the hexoctahedral class (class m3̄m, cubic). The seven crystal systems — triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic — group these classes by their minimum symmetry requirements, and the forms available within each system reflect that symmetry. Gemmologically important minerals are distributed across all seven systems: feldspar (triclinic), orthoclase (monoclinic), topaz (orthorhombic), zircon (tetragonal), corundum and tourmaline (trigonal), beryl (hexagonal), and diamond, spinel, and garnet (cubic).

The concept of the general form is particularly useful in systematic mineralogy because it defines the maximum multiplicity of faces for a class and thus the class's full name. In teaching gemmology, the general form also provides an intuitive entry point: if one can identify which general form a crystal approximates, one has already constrained the crystal system and narrowed the list of candidate species considerably.

Further Reading

• GIA Gems & Gemology — Crystal Forms in Gem Identification
• International Gem Society — Crystallography: An Introduction for Gemologists