The concave cut is a faceting style in which individual facets are ground as inwardly curving surfaces rather than the flat planes that define every conventional faceting style from the brilliant to the step cut. The result is a gemstone whose interior appears to recede and expand simultaneously — a sculptural, almost liquid quality that has earned the technique a devoted following among collectors and studio jewellers since its systematic development in the early 1990s. Because each curved facet demands specialised equipment, painstaking setup, and significantly longer cutting time than standard flat faceting, concave-cut stones command substantial premiums and are produced in comparatively small numbers worldwide.

Origins and Development

The concave cut is most closely associated with American lapidary Doug Hoffman, who developed and patented the technique in the early 1990s after years of experimentation with modified faceting machinery. Hoffman's central insight was that a facet ground on a curved surface — essentially a section of a cylinder or cone rather than a plane — would interact with light in fundamentally different ways from a flat facet, generating internal reflections and refractions unavailable to conventional cutting. He subsequently founded the International School of Gem Arts to teach the method, and the concave cut became one of the most discussed innovations in late-twentieth-century lapidary arts.

Earlier lapidaries had occasionally experimented with curved surfaces in cabochon and fantasy cutting, and the broader category of Freiform or free-form cutting in Germany explored sculptural approaches to gemstone shaping. Hoffman's contribution was to apply curved geometry systematically to faceted stones — stones intended to return light through their crowns — and to engineer the tooling required to do so with repeatability and precision.

How Concave Faceting Works

In conventional faceting, a gemstone is held on a dop stick and presented to a rotating flat lap at precisely controlled angles; each facet is ground and polished as a true plane. Concave faceting replaces the flat lap with a cylindrical or conical rotating tool — sometimes described as a concave lap or router bit — whose curved surface grinds a corresponding concave depression into the stone. The lapidary must control not only the angle of presentation but also the lateral position of the stone relative to the tool's axis, adding a dimension of mechanical complexity absent from flat faceting.

The geometry of each concave facet can be varied: a shallow curve produces a subtle effect, while a deeper curve creates more pronounced internal reflections and a stronger three-dimensional impression. Matching the curvature consistently across all facets of a given stone — so that the finished gem reads as coherent rather than chaotic — requires both precise tooling and considerable skill. Cutting times for a concave-cut stone are typically several times longer than for an equivalent flat-faceted stone of the same size and design.

Optical Effects

The distinctive appearance of a concave-cut gemstone arises from the way curved facets redirect light. A flat facet acts as a small mirror or refraction surface with a single, well-defined angle of reflection; a concave facet, by contrast, presents a continuously varying angle across its surface, spreading reflected and refracted light into a fan or cone rather than a point. The practical consequences include:

• Enhanced scintillation: As the stone or the light source moves, the curved facets produce a rolling, shifting play of reflections rather than the discrete flashes of a brilliant-cut stone.
• A "wet" or liquid appearance: The continuous gradation of reflected light across each facet gives the stone a visual depth reminiscent of a polished water surface — an effect particularly prized in pale or colourless material.
• Three-dimensional internal structure: Because the curved facets create reflections at angles unavailable to flat faceting, the interior of the stone appears to contain layers and volumes rather than the relatively flat mirror-like return of a standard cut.
• Reduced windowing: In some designs, the curved pavilion facets can reduce the tendency of a shallow stone to show a transparent "window" through its centre, though this depends heavily on the specific geometry employed.

These effects are most legible in transparent, lightly coloured or colourless material. In deeply saturated stones, the colour itself can mask the subtler optical contributions of the curved facets.

Suitable Gem Materials

Concave cutting is most effective — and most commonly encountered — in gem materials that are transparent, relatively pale in colour, and available in sizes large enough to allow the curved facets to read clearly. Frequently used materials include:

• Quartz varieties (rock crystal, smoky quartz, amethyst, citrine): widely available in large, clean crystals; the modest refractive index of quartz (approximately 1.544–1.553) is partly compensated by the enhanced light interaction of the curved facets.
• Topaz: high clarity, good hardness (Mohs 8), and a refractive index of approximately 1.619–1.627 make it well-suited to the technique.
• Beryl (aquamarine, heliodor, morganite, goshenite): the typically pale, clean crystals of these varieties respond well to concave faceting's light-spreading effects.
• Tourmaline and kunzite: used by skilled cutters where crystal size and clarity permit.

Concave cutting in corundum, spinel, or other harder and more valuable materials is far less common, partly because the greater hardness increases cutting time and tool wear substantially, and partly because the premium material cost raises the stakes of any error. When concave-cut sapphires or rubies do appear, they are typically the work of specialist cutters and command exceptional prices.

Design Variations

Concave faceting is not a single cut but a technique applicable to a wide range of designs. Hoffman and subsequent practitioners have developed numerous named patterns, including concave versions of the brilliant, the oval, the cushion, and various fantasy or freeform outlines. Some designs apply concave facets only to the pavilion, leaving the crown conventionally flat-faceted; others apply the technique throughout. Hybrid approaches — combining concave facets with flat facets, or with polished curved surfaces reminiscent of cabochon work — are also documented.

A related category is the concave star cut, in which the star facets of a modified brilliant are replaced with concave surfaces, producing a pronounced starburst of internal reflections. Fantasy cutters working in the German Idar-Oberstein tradition have incorporated concave elements into sculptural gem carvings, blurring the boundary between faceting and gem carving.

In the Trade

Concave-cut gemstones occupy a specialist niche within the broader market for precision-cut and designer-cut stones. They are most commonly encountered through individual lapidary artists, small cutting studios, and specialist gem dealers rather than through mainstream commercial channels. The stones appear regularly at gem and mineral shows — particularly the Tucson Gem and Mineral Show — and are collected both as finished gems and as lapidary art objects in their own right.

Pricing reflects the labour intensity of the technique. A concave-cut aquamarine or topaz of moderate size may command two to four times the price of an equivalent stone in a standard brilliant or emerald cut, depending on the complexity of the design and the reputation of the cutter. Provenance — knowing the specific lapidary who cut a stone — carries unusual weight in this market segment, more so than in the trade for commercially cut material.

From a jewellery-design perspective, concave-cut stones present both opportunities and challenges. Their sculptural depth and unusual light behaviour make them compelling centrepieces in bespoke and studio jewellery, but their non-standard geometries and sometimes irregular profiles require custom settings. The curved facets are also somewhat more susceptible to chipping along their edges than flat facets, particularly in stones of lower hardness, and this should be considered in setting design.

Further Reading

• Gems & Gemology — GIA's quarterly journal, gia.edu
• Fantasy Cutting — International Gem Society, gemsociety.org