The half-moon diamond is a fancy-shaped diamond fashioned to approximate a semicircle or, in some interpretations, a gentle crescent, with one straight or slightly curved edge and one fully rounded edge. It occupies a specialised niche within the broader category of fancy-shape diamonds, sitting outside the parameters of the standard round brilliant and therefore receiving no cut grade on GIA grading reports. Its principal role in the trade is as a flanking side stone, where its geometry allows it to nestle cleanly against rectangular, square, or oval centre stones, creating a visually coherent and architecturally satisfying composition. Though rarely encountered as a solitaire, the half-moon diamond rewards close examination: a well-cut example displays lively brilliance across its curved face and a clean, symmetrical outline that speaks to considerable skill at the cutting wheel.

Geometry and Proportions

The defining characteristic of the half-moon diamond is its bilateral asymmetry of outline: a flat or gently curved chord on one side — typically the edge that abuts the centre stone or the mount — and a convex arc on the other. In the strictest interpretation, the stone is a true semicircle, with the chord forming an exact diameter. In practice, cutters sometimes introduce a slight curvature to the flat edge, producing what the trade occasionally calls a crescent or curved half-moon, which can follow the shoulder of an oval or cushion centre stone more naturally.

Depth ratios for half-moon diamonds generally fall in the range of 55 to 70 per cent of the stone's diameter, though there is no universally codified standard, as there is for the round brilliant. The table percentage, crown angle, and pavilion depth are assessed by individual cutters and laboratories on a case-by-case basis. Symmetry is paramount: the two halves of the stone, divided by an imaginary perpendicular bisector of the chord, should mirror one another precisely. Any deviation — a lopsided arc, an off-centre culet, or uneven facet placement — is immediately apparent when the stones are set as a matched pair.

Faceting Approaches

There is no single canonical faceting scheme for the half-moon diamond, and different cutting houses apply varying arrangements. The most common approach adapts the brilliant-cut facet pattern — kite-shaped bezel facets on the crown, triangular and kite-shaped pavilion facets — to the semicircular outline. The result is a stone that, viewed face-up, displays a pattern of light return broadly comparable to that of a brilliant-cut, though the geometry inevitably introduces some compromise at the curved perimeter, where facets must be adjusted to maintain the outline.

A second approach borrows from the step-cut tradition, using rows of parallel, trapezoidal facets on both crown and pavilion. Step-cut half-moons tend toward a quieter, more reflective character — closer in optical personality to an emerald cut than to a round brilliant — and are occasionally chosen when the centre stone is itself a step cut, such as an Asscher or emerald-cut diamond, where tonal consistency across the composition is desired.

The bow-tie effect, a dark shadow across the centre of the stone caused by light escaping through the pavilion rather than returning to the eye, is a known risk in elongated and asymmetric fancy shapes. In half-moon diamonds, a pronounced bow-tie is most likely to appear when the pavilion is too shallow or when the facet angles are poorly optimised for the outline. Assessing the bow-tie requires viewing the stone under varied lighting conditions and at different angles; it cannot be reliably inferred from proportions alone.

Grading and Laboratory Reports

Because the half-moon diamond falls outside the round brilliant parameters, the Gemological Institute of America (GIA) does not assign a cut grade on its grading reports for stones of this shape. Reports will, however, document the standard 4Cs — colour, clarity, carat weight, and a descriptive clarity grade — along with a shape and cutting style designation. The absence of a cut grade places greater responsibility on the buyer and their gemmologist to evaluate proportions, symmetry, and optical performance independently.

Symmetry is assessed on GIA reports using the standard descriptive scale (Excellent, Very Good, Good, Fair, Poor), and for matched pairs — the most common commercial configuration — symmetry grades should ideally match between the two stones. Polish is likewise graded on the same scale and reflects the quality of the facet surfaces under magnification.

Other major laboratories, including the International Gemological Institute (IGI) and Hoge Raad voor Diamant (HRD), follow comparable conventions: shape and cutting style are described, but no cut grade is issued for fancy shapes of this kind.

Use in Jewellery

Half-moon diamonds are almost exclusively deployed as side stones, set in pairs flanking a centre stone. The most architecturally coherent pairings place the flat chord of each half-moon directly against the shoulder of the centre stone, so that the curved arc faces outward toward the shank. This arrangement is particularly effective with:

• Oval centre stones, where the curved chord of the half-moon can be cut to echo the oval's own curvature, producing a seamless visual flow.
• Cushion-cut diamonds and coloured stones, where the half-moon's rounded arc softens the overall composition.
• Rectangular cuts — emerald cuts, Asscher cuts, radiant cuts — where the straight chord of the half-moon aligns cleanly with the centre stone's long edge.

In high jewellery, half-moon diamonds occasionally appear in more complex configurations: as elements within pavé-set halos, as shoulder stones in multi-row settings, or as accent stones in brooches and earrings where their distinctive outline contributes to a geometric or Art Deco-inflected design vocabulary. The shape has an affinity with the graphic precision that characterised jewellery design of the 1920s and 1930s, and contemporary designers working in a retro or architectural idiom frequently return to it.

Carat weights in commercial use typically range from approximately 0.15 to 1.50 carats per stone, with matched pairs in the 0.25–0.75 carat range per stone representing the most common market configuration. Stones above 1.00 carat per side are relatively uncommon and command a meaningful premium, both for the rarity of suitable rough and for the skill required to cut a large, symmetrical half-moon with satisfactory optical performance.

Sourcing and the Cutting Process

Half-moon diamonds are cut from rough that would not yield an economically optimal round brilliant or other standard fancy shape. Triangular or irregularly shaped rough crystals, macles (twinned octahedral crystals), and off-cuts from the production of larger stones are common starting points. The cutter must balance yield — maximising the carat weight recovered from the rough — against the geometric precision required for the finished outline and the optical requirements for acceptable brilliance.

Cutting centres in Surat, Antwerp, and Tel Aviv all produce half-moon diamonds, with Surat handling the majority of commercial-grade production and Antwerp and Tel Aviv associated with higher-quality, better-proportioned stones destined for fine jewellery. As with all fancy shapes, the quality of the finished stone is heavily dependent on the individual cutter's skill and the standards of the house for which they work.

Valuation Considerations

Half-moon diamonds are valued using the same fundamental framework as other fancy-shape diamonds — colour and clarity grades, carat weight, and an assessment of cutting quality — but the absence of a standardised cut grade introduces subjectivity into the last of these factors. Buyers and valuers should pay particular attention to:

• Symmetry grade on the laboratory report, and visual confirmation of symmetry in the physical stone.
• The presence and severity of any bow-tie effect, assessed under varied lighting.
• The precision of the outline: the arc should be smooth and continuous, and the chord should be straight (or, in crescent cuts, consistently curved).
• For pairs, the degree of match in colour, clarity, proportions, and overall appearance between the two stones.

A well-matched pair of half-moon diamonds in D–F colour and VS clarity, with Excellent or Very Good symmetry and no visible bow-tie, will command prices broadly comparable to other well-cut fancy shapes of equivalent weight and quality, though the half-moon's specialised application means that demand — and therefore liquidity — is narrower than for more versatile shapes such as the oval or cushion.

Further Reading

• GIA: Diamond Quality Factors — gia.edu
• GIA: The 4Cs of Diamond Quality — gia.edu