The bombé cut — from the French for "vaulted" or "convex" — is a cabochon form characterised by an exceptionally pronounced, steeply arched dome that rises well above the girdle plane. Where a standard cabochon presents a gently rounded upper surface, the bombé profile is emphatically three-dimensional: the crown curves sharply upward from the girdle, creating a bold, almost spherical upper hemisphere. This exaggerated convexity is not merely aesthetic. In optical-phenomenon stones — star sapphires, star rubies, cat's-eye chrysoberyl, alexandrite cat's-eyes, and chatoyant tourmalines — the geometry of the dome directly governs the sharpness, centring, and brilliance of the phenomenon itself. The bombé cut is therefore both a lapidary discipline and an optical instrument.

Geometry and Proportions

A standard cabochon is generally described as having a dome height roughly equal to one-third to one-half of the stone's width. The bombé cut departs from this convention by pushing the dome height considerably higher — often approaching or exceeding two-thirds of the width, and in extreme examples nearly equalling it. The result is a profile that, viewed from the side, resembles the upper portion of a sphere or a pronounced ellipsoid. The base may be flat or very slightly convex, and the girdle is typically kept narrow to maximise the impression of volume.

This geometry has direct consequences for the lapidary. A greater dome height demands substantially more rough depth, meaning that the cutter must begin with a deeper piece of material than a standard cabochon of equivalent face diameter would require. Weight retention — the proportion of rough that survives as finished stone — is correspondingly lower. For a commodity material this would be an unattractive trade-off, but in high-quality asteriated or chatoyant rough, where the optical phenomenon is the primary value driver, the sacrifice in yield is commercially justified by the superior result.

Optical Rationale: Asterism and Chatoyancy

The optical phenomena that the bombé cut is designed to serve arise from oriented microscopic inclusions within the host crystal. In star sapphires and star rubies, needle-like rutile inclusions align along the crystallographic axes of the corundum host, typically in three sets at 60° to one another, producing a six-rayed star under a single light source. In cat's-eye chrysoberyl, parallel fibrous or tubular inclusions scatter light into a single luminous band across the dome.

In both cases, the curvature of the dome acts as a focusing surface. The inclusions must lie parallel to the base of the stone — a requirement the lapidary addresses by orienting the rough so that the crystallographic axis runs perpendicular to the table — and the dome then converges reflected light from those inclusions into a sharp, mobile point or band at the apex of the stone. The steeper the dome, the more tightly the phenomenon is concentrated. A low or flat dome diffuses the star or eye across a broad, indistinct area; a well-proportioned bombé dome pulls it into a crisp, well-defined image that moves convincingly across the surface as the light source shifts.

This is why gemmologists and experienced buyers assess the dome height of a star stone or cat's-eye as a primary quality criterion alongside colour, transparency, and the sharpness of the phenomenon itself. A fine Kashmir star sapphire or a top-grade Sri Lankan cat's-eye chrysoberyl cut with an insufficiently high dome will never realise its optical potential, regardless of the quality of the rough.

Materials Suited to the Bombé Form

The bombé cut is most closely associated with the following gem materials:

• Star sapphire and star ruby (corundum): The classic application. Stones from Sri Lanka (formerly Ceylon), Burma (Myanmar), and Thailand have historically been cut in this form, with Sri Lankan material — particularly the blue and grey varieties — producing some of the most celebrated high-domed star stones in the record. The Black Star of Queensland, one of the largest star sapphires known, exemplifies the bombé profile taken to an extreme scale.
• Cat's-eye chrysoberyl: The finest examples, predominantly from Sri Lanka and Brazil, are routinely cut as high-domed ovals or rounds to maximise the sharpness of the chatoyant band. The ideal cat's-eye chrysoberyl dome is steep enough to produce the celebrated "milk and honey" effect — a sharp division between the illuminated and shadowed halves of the stone — which requires precise dome geometry.
• Chatoyant tourmaline: Certain pink, green, and blue tourmalines from Brazil, Mozambique, and Afghanistan display strong chatoyancy and benefit from the bombé form.
• Alexandrite cat's-eye: A relatively rare combination of colour-change and chatoyancy, found in material from Sri Lanka and Brazil, where both phenomena are best served by a high dome.
• Moonstone: High-quality adularescent moonstones from Sri Lanka and Myanmar are sometimes cut in the bombé style to deepen the adularescence and give the stone a more imposing physical presence.
• Cymophane: The chatoyant variety of chrysoberyl is essentially synonymous with the cat's-eye form and shares the same cutting rationale.

Lapidary Technique

Cutting a bombé cabochon requires careful pre-forming of the rough to establish the correct orientation before grinding begins. The lapidary first identifies the direction of the inclusions — typically by examining the rough under a strong directional light source — and marks the base of the stone perpendicular to the inclusion axis. The rough is then dopped (attached to a dop stick) with the base aligned, and the dome is ground progressively through coarser to finer grits, maintaining a symmetrical, steeply arched profile throughout.

Achieving a truly symmetrical bombé dome by hand requires considerable skill; any asymmetry in the dome will cause the star or eye to migrate off-centre or appear lopsided. The final polishing stage, typically using cerium oxide or diamond paste on a leather or felt lap, must preserve the curvature established in grinding without flattening the apex — a common error that immediately degrades the phenomenon.

In commercial cutting centres such as Ratnapura (Sri Lanka) and Chanthaburi (Thailand), experienced cutters develop an intuitive feel for the optimal dome height for a given piece of rough, balancing the competing demands of weight retention and optical performance. In fine-quality material, optical performance invariably takes precedence.

The Bombé Cut in Jewellery Design

Beyond its optical function, the bombé cut imparts a strong sculptural presence to a jewellery piece. The pronounced dome rises assertively from the setting, giving rings, brooches, and pendants a bold, three-dimensional quality that flat or low-profile stones cannot achieve. This visual weight made the bombé form particularly favoured during the mid-twentieth century, when large star sapphires and cat's-eye chrysoberyls set in platinum or yellow gold were prized centrepieces of important jewellery. Major auction houses regularly present bombé-cut star corundum of significant size in their jewellery sales, where the combination of fine colour, sharp phenomenon, and imposing dome height commands the highest premiums.

Settings for bombé cabochons must accommodate the height of the dome. Bezel settings — in which a band of metal wraps the girdle — are the most secure and most traditional choice, protecting the relatively exposed apex of a high dome from impact. Claw or prong settings are less common for very high domes but are used when the designer wishes to maximise light entry through the sides of the stone.

Grading Considerations

When evaluating a bombé-cut stone, gemmologists and buyers consider the following in conjunction with standard cabochon quality factors:

• Dome symmetry: The dome should be evenly curved in all directions from the apex; asymmetry displaces the phenomenon from centre.
• Apex centring: The highest point of the dome should coincide with the geometric centre of the base outline.
• Phenomenon sharpness: The star or eye should be well-defined and mobile, not diffuse or stationary.
• Dome height relative to width: Higher domes generally produce sharper phenomena but require deeper, more valuable rough; the ratio is a proxy for cutting quality and material quality alike.
• Base flatness: A flat, well-finished base facilitates secure setting and indicates careful workmanship.

Further Reading

• GIA: Star Sapphire
• GIA: Cat's-Eye Chrysoberyl
• International Gem Society: Cabochon Cutting