A pentagon cut is a faceted or cabochon gemstone whose plan-view outline is a five-sided polygon. The form is far less common in commercial cutting than the round, oval, cushion, princess, and emerald cuts, and it appears in the trade for two principal reasons: as a designer choice in pieces where the unusual silhouette is the point, and as a yield-driven adaptation when the cutter is faced with rough that does not lend itself cleanly to a more standard outline.

The geometry

A regular pentagon has five sides of equal length and five interior angles of 108 degrees. In gemstone form, perfectly regular pentagons are rare; most pentagonal cuts are slightly elongated or asymmetric to suit the rough or the design. Faceting patterns vary widely. One common approach adapts the kite or shield faceting layout, with brilliant-style crown facets stepped around the centre line and a corresponding pavilion designed to return light efficiently across the asymmetric outline. Another approach uses step-cut crown and pavilion facets running parallel to the five sides, producing a flatter, more architectural appearance closer in feel to the emerald cut.

Pentagonal outlines present cutting challenges that the more familiar shapes do not. The five-fold symmetry has no obvious axis of optical symmetry that maps cleanly onto the standard brilliant geometry, and the cutter must work harder to suppress windowing in the centre and to balance brightness across the asymmetric belly of the stone.

When the cut appears

Designer jewellery is the principal market for pentagonal stones. A jeweller working on a piece with an unusual focal point — a pentagonal bezel, a five-pointed star setting, a piece with explicit numerical symbolism — may commission a pentagon-cut centre stone to suit. Sapphire, tourmaline, garnet, aquamarine, and quartz are the species most often seen in pentagonal form, both because they are available in clean rough at sizes large enough to permit unusual outlines and because their refractive indices and dispersion suit the cutter's adaptive faceting. Diamonds are seldom cut as pentagons, since diamond's value structure rewards the standardised outlines.

The yield case arises when a piece of rough is shaped in such a way that a five-sided outline preserves more carats than a forced fit to a regulation oval or cushion. A skilled cutter examining a piece of awkward rough sometimes finds that the pentagon is the answer that minimises waste, and the resulting stone enters the market on its own terms.

In the trade

Pentagon cuts trade at a discount per carat to comparable round or oval stones of similar species and quality, reflecting the limited setting market and the additional design effort required of the jeweller using them. Buyers should evaluate the stone on the standard parameters — colour, clarity, brilliance, symmetry of the outline, polish — and then consider whether the design context for which they are buying actually wants a five-sided outline. For a stone selected on its own merits, the discount can represent good value; for a stone shopped against the standard round-brilliant market, the comparison is misleading.

Further reading

• GIA — gemstone cuts and shapes
• International Gem Society — fancy gemstone shapes
• GIA Gems & Gemology — coloured stone cutting articles