A crystal form is a group of matching faces that a crystal's symmetry produces together. A cube is one form with six faces, and an octahedron is another with eight. Most real crystals combine two or more forms, and many grow as twins, where two or more crystals share a common plane or axis in a fixed orientation. Forms and twins matter to gemologists because they reveal how a crystal grew and help identify rough before it is cut.
Faces, edges and symmetry
The flat outer surfaces of a natural crystal are its faces. They are not random. Each face lies parallel to a plane of atoms in the crystal lattice, so the angles between faces stay the same from one crystal of a mineral to the next, even when the crystals differ in size or in how evenly they grew.
Symmetry describes how a crystal repeats itself. Crystallographers use three tools to describe it:
- An axis of symmetry is a line around which the crystal looks the same after a partial turn. A twofold axis repeats every half turn, and a fourfold axis every quarter turn.
- A mirror plane divides the crystal into two halves that are mirror images.
- A center of symmetry means every face has a matching parallel face on the opposite side.
Combining these elements gives 32 crystal classes, which group into the seven crystal systems. A cube has the highest symmetry possible: three fourfold axes, four threefold axes, six twofold axes and nine mirror planes.
Closed and open forms
A closed form is a set of faces that encloses space by itself. The cube, octahedron and dodecahedron are closed forms, and a crystal can consist of a single closed form. Diamond octahedra are a good example.
An open form does not enclose space on its own. A prism is a set of faces parallel to one axis, so it needs other faces at the ends. A pinacoid is a pair of parallel faces, such as the flat end of an emerald crystal. A pyramid is a set of faces that meet at a point at one end only. Open forms always appear together with other forms.
Common crystal forms in gems
| Form | Faces | Where you see it |
|---|---|---|
| Cube | 6 square faces | Fluorite, pyrite, rare diamonds |
| Octahedron | 8 triangular faces | Diamond, spinel, magnetite, fluorite |
| Rhombic dodecahedron | 12 rhomb-shaped faces | Garnet, some diamonds |
| Trapezohedron | 24 kite-shaped faces | Garnet |
| Hexagonal prism | 6 faces parallel to the long axis | Beryl, apatite, quartz |
| Trigonal prism | 3 faces parallel to the long axis | Tourmaline |
| Pinacoid | 2 parallel faces | Ends of beryl prisms, flat corundum plates |
| Pyramid or dipyramid | Faces meeting at one or two points | Zircon, sapphire, quartz terminations |
| Rhombohedron | 6 rhomb-shaped faces | Calcite, ends of corundum and quartz crystals |
Cubic forms
The cubic system has five forms that show up most often in gem rough: the cube, the octahedron, the dodecahedron, the trapezohedron and, less often, the tetrahexahedron. Diamond most often grows as octahedra, sometimes as cubes, and its faces are often rounded by partial dissolution as the crystal rose toward the surface. Spinel forms sharp, glassy octahedra. Garnet forms dodecahedra and trapezohedra, or a combination of both.
Prisms and terminations
Gems from the other systems are often prisms with something else on the ends. An emerald crystal is usually a hexagonal prism capped by a flat pinacoid. Quartz combines a hexagonal prism with two rhombohedra that together look like a six-sided pyramid. Zircon combines a four-sided prism with a pyramid at each end. Sapphire often forms a hexagonal dipyramid, a tapering barrel with points at both ends.
Striations
Fine parallel lines on crystal faces are called striations. They form when two forms alternate rapidly during growth. Tourmaline prisms are striated along their length, quartz prism faces are striated across, and pyrite cubes often carry fine lines that run in different directions on neighboring faces. The direction of striations is a useful identification clue for rough.
Twinning
A twinned crystal is two or more individual crystals of the same mineral joined in a symmetrical, rule-based way. The parts share some atoms along the boundary, called the composition plane, and one part relates to the other by a reflection or a rotation. Twins form during growth, or when a crystal changes structure as it cools, or when it is deformed by stress. Each regular pattern is called a twin law, often named for a mineral or a place.
Contact twins
In a contact twin, two crystals meet along a single flat composition plane, like a crystal and its mirror image sharing a wall. The diamond macle is the best-known example.
Penetration twins
In a penetration twin, two crystals grow through each other and appear to cross. Fluorite and pyrite form interpenetrating cubes. Staurolite, a brown silicate, grows as cross-shaped twins sold as "fairy crosses" in folk tradition.
Repeated twins
Repeated twins (polysynthetic twins) form when many thin layers alternate in twin orientation. Plagioclase feldspars such as labradorite show this as fine parallel lines on cleavage surfaces. In corundum, repeated twinning can create parting planes and the crossing lines that gemologists see under magnification in some ruby and sapphire. When three or more individuals repeat around a center, the result is called a cyclic twin. Chrysoberyl twins this way into six-sided "trillings," a habit well known in alexandrite rough that looks hexagonal even though chrysoberyl is orthorhombic.
The diamond macle
A macle is a flattened, triangular contact twin of two diamond octahedra rotated relative to each other. Macles often show a herringbone line along the twin plane and can be hard to polish because the grain direction changes at the boundary. Diamond cutters often saw macles into triangular stones, and macles are popular in rough diamond jewelry.
Japan law twins in quartz
A Japan law twin is two flat quartz crystals joined at an angle of about 84 degrees, forming a heart or V shape. The name comes from fine specimens found in Japan in the 19th century. They are prized by collectors and appear in specimen jewelry. Quartz also twins on the Dauphine and Brazil laws, which are usually invisible from outside but matter in quartz used for electronics. In ametrine, twinning zones help explain why the purple and yellow colors sit in separate sectors.
Why twinning matters in gems
Twinning affects both cutting and identification. Twin planes in diamond change the grain direction, so they can resist polishing and make a stone harder to cut. Twinning wisps, plane-like groups of inclusions along a twin boundary, are graded as clarity features and appear in the diamond inclusion library. In corundum, twin lamellae and the parting they cause can be a weak direction in a stone. Under crossed polarizers, some twinned gems show a pattern of light and dark sectors, which a gemologist can read with a polariscope from the gemological instruments toolkit.
Frequently asked questions
What is the difference between a crystal form and a crystal habit?
A form is a set of faces related by symmetry, such as an octahedron. Habit is the general shape a mineral tends to grow in, which may combine several forms.
What is a macle?
A macle is a flat, triangular twinned diamond crystal made of two octahedra joined along a shared plane. Cutters often turn macles into triangular stones.
What is a Japan law twin?
It is a pair of flat quartz crystals joined at about 84 degrees in a heart or V shape, named for classic specimens from Japan.
Do twinned crystals look like two crystals?
Sometimes. Penetration and contact twins can be easy to spot, but repeated and some cyclic twins look like a single crystal from outside and show their twinning only under magnification or polarized light.