Prism — The Open Tube of Crystal Faces
Prism — The Open Tube of Crystal Faces
A set of three or more parallel crystal faces, named by the count and fundamental to gem-rough identification
In crystallography, a prism is a set of three or more parallel crystal faces that together form an open tube running the length of the crystal. Prisms do not close the form; they are combined with pinacoids, pyramids, or other terminal faces to make a complete crystal. Prisms are named by the number of parallel faces, with trigonal, tetragonal, hexagonal, and ditetragonal prisms among the most familiar in gem species. The form is fundamental to crystal-habit description and is one of the first features the field collector and the gem cutter use to identify rough material.
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Geometry of the prism form
A crystal prism is defined by a set of faces that all share a common direction, the prism axis, and that together enclose an open elongated shape. The prism faces are parallel to the prism axis but inclined to one another at angles dictated by the crystal symmetry. A trigonal prism has three faces at one hundred and twenty degrees, a tetragonal prism has four faces at ninety degrees, a hexagonal prism has six faces at sixty degrees, and a ditetragonal prism has eight faces in two alternating sets.
Because the prism alone leaves the ends of the crystal open, every real crystal terminates the prism with another form, most often a pinacoid (a pair of parallel basal faces), a pyramid (a set of inclined faces meeting at a point), or a more complex termination involving multiple forms. The combination of prism plus termination defines the overall habit of the crystal and is what the experienced eye reads when picking up a piece of rough.
Prismatic forms in common gem species
The hexagonal prism is one of the most familiar crystal forms in the gem trade because it is the dominant prism in beryl and in corundum. Beryl crystals, including emerald, aquamarine, morganite, and heliodor, grow as hexagonal prisms terminated either by a flat pinacoid (giving the classic hexagonal-pencil shape) or by a low pyramid. Corundum crystals, including ruby and sapphire, grow as hexagonal prisms terminated by basal pinacoids or pyramids and often modified by rhombohedral faces, giving the characteristic barrel-shape of unmodified ruby and sapphire rough.
The trigonal prism appears in the trigonal members of the corundum group and in several other species. The tetragonal prism is dominant in zircon and in the rutile group, and the ditetragonal prism is common in zircon. Tourmaline grows as a trigonal or ditrigonal prism with a striated surface that is one of the species' most recognisable identification features.
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Quartz, despite its complex internal trigonal symmetry, presents in everyday crystal form as a hexagonal prism terminated by alternating large and small rhombohedral faces, the well-known six-sided pencil shape. The pseudohexagonal appearance has occasionally led to misidentification at the field level but is resolved on closer examination by the alternating size of the terminating rhombohedra.
Prism striations and growth markers
Many prismatic gem crystals carry visible striations on the prism faces, running parallel to the prism axis. Striations result from rapid stacking of crystal layers during growth and are diagnostic for several species. The vertical striations on tourmaline run along the prism axis, and this is one of tourmaline's strongest identification features in the rough.
Topaz, beryl, and corundum can also show prism striations, although less reliably as identification features. The presence and pattern of striations help the cutter understand the orientation of the crystal and plan the cutting strategy; for pleochroic species, the prism axis usually corresponds to one of the principal optic directions, and the cutter aligns the cut to that axis to control face-up colour.
Prismatic versus other crystal habits
A prismatic habit is one of several elongation patterns a crystal can show. Acicular crystals are needle-like with very small cross-section relative to length; tabular crystals are flat with one direction much shorter than the other two; equant crystals have all three directions roughly equal. The prismatic habit sits between equant and acicular, with significant elongation along the prism axis but a substantial cross-section perpendicular to it.
The boundary between prismatic and acicular is a matter of degree rather than of crystallographic type. A long thin tourmaline crystal whose length is twenty times its width is described as acicular; a stubby tourmaline crystal whose length is two or three times its width is described as prismatic. Both forms have the same crystallographic prism faces and the same internal symmetry; only the relative dimensions differ.
Diagnostic use in the field
The prism is one of the first features a field collector or a gem rough buyer uses to narrow the species identification of an unknown crystal. The number of prism faces, the angles between them, the presence and direction of any striations, and the form of the termination all contribute to a tentative species identification before any laboratory test is run. The technique is taught in introductory crystallography courses and remains the working method for most rough buying done outside the laboratory.
For confirmed identification, of course, the prism geometry alone is insufficient; refractive index, specific gravity, and spectroscopy are required. But the prism is the visual cue that suggests where to start, and a buyer who can read prism geometry quickly will work through a parcel of rough much faster than one who has to test every stone from scratch.
In the trade
For the gem cutter and the rough buyer, the prism is the working geometry of the crystal. Cutting yield, orientation for colour, and final cut shape are all influenced by the prism faces of the original rough. Even after the crystal is cut and the prism is destroyed, the orientation of the optic axis relative to the original prism axis remains a feature of the finished gem and shows up in pleochroism, in the position of inclusions, and in the pattern of any growth zoning visible under magnification.
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