The half-moon cut — also called the lunette — is a faceted gemstone shape whose girdle outline describes a semicircle: one edge is straight, the opposite edge is curved, and the two meet at right angles at each corner. It is employed almost exclusively as a side stone, designed to nestle flush against the flat facet of a central emerald-cut, oval, or cushion-cut stone, creating a seamless architectural line across the finger. Though modest in individual carat weight — typically ranging from 0.20 to 1.00 ct per stone — the half-moon cut demands a level of precision that rivals far larger fancy shapes, and a well-matched pair can dramatically elevate the visual presence of a three-stone ring or a flanked solitaire.

Geometry and Faceting Structure

The defining characteristic of the half-moon is the tension between its two contrasting edges. The straight edge — the diameter of the semicircle — must be cut to an exact length to align with the table edge or girdle of the centre stone it accompanies. Any deviation in that measurement, even a fraction of a millimetre, produces a visible gap or an awkward overlap in the finished mounting. The curved edge follows a true arc, and the precision of that arc determines whether the stone reads as a clean half-circle or as an irregular, lopsided form.

Faceting conventions for the half-moon are not rigidly standardised across the trade, and cutters exercise some latitude, but the most widely adopted approach is a mixed cut: step-cut facets on the pavilion and a brilliant-style arrangement on the crown. The pavilion typically carries two to four rows of elongated, trapezoidal step facets that run parallel to the straight edge, maximising depth and minimising windowing across the broad, flat belly of the stone. The crown, by contrast, uses triangular and kite-shaped facets radiating from a small table, introducing the scintillation and brightness that pure step cutting alone would suppress. Some cutters apply a fully step-cut treatment to both crown and pavilion — a configuration closer to the baguette tradition — while others use a brilliant pavilion with a step crown, reversing the usual hierarchy. Each approach produces a different optical character, and buyers should examine stones under both diffuse and directional light to assess which variant suits the intended design.

The table facet itself is generally kept narrow relative to the overall width of the stone, partly to preserve crown height and partly because a very large table on a curved outline tends to produce a glassy, lifeless appearance. Ideal depth percentages are broadly analogous to those of the baguette or tapered baguette — roughly 60–70 per cent of width — though the curved outline complicates direct comparison with rectangular step cuts.

Symmetry and the Windowing Problem

Windowing — the phenomenon in which a pale, washed-out zone appears at the centre of a faceted stone, allowing the viewer to see straight through to the surface beneath — is the principal quality hazard in half-moon cutting. It arises when pavilion angles are too shallow, a risk that is heightened in the half-moon because the stone's broad, flat profile encourages cutters to sacrifice depth in order to retain carat weight from the rough. A windowed half-moon reads as dull and inert beside a well-proportioned centre stone, and the contrast is particularly unflattering in transparent coloured stones such as sapphire, aquamarine, or spinel, where the colour itself is expected to carry visual weight.

Symmetry is the second critical variable. A matched pair of half-moons — the standard requirement for flanking a centre stone — must be mirror images of one another in outline, depth, and facet arrangement. Discrepancies in the arc radius, in the length of the straight edge, or in the angle of the corners produce a restless, unbalanced appearance in the finished piece that no amount of skilled setting can fully conceal. For this reason, reputable cutters produce half-moons in calibrated pairs from the outset, working from matched pieces of rough where possible.

Historical and Design Context

The half-moon cut rose to particular prominence during the Art Deco period (approximately 1920–1939), when jewellers across Paris, London, and New York embraced geometric precision as an aesthetic ideal. The period's characteristic three-stone compositions — a central emerald-cut diamond flanked by two half-moons — expressed the era's preference for rectilinear clarity tempered by a single curved counterpoint. Cartier, Van Cleef & Arpels, and Boucheron all produced pieces in this configuration, and the form has remained closely associated with the Art Deco aesthetic ever since.

The half-moon also appears in Art Deco and Retro-period brooches, where pairs of stones are set point-to-point to form a full circle, and in earrings where a single stone is oriented with the curved edge downward to suggest a hanging drop. In contemporary jewellery, the cut has experienced renewed interest as designers seek alternatives to the ubiquitous round brilliant for side-stone positions, and as the broader market for fancy-cut coloured stones has expanded. Sapphire, aquamarine, tourmaline, and spinel are among the coloured species most frequently encountered in half-moon form; diamond half-moons remain the dominant choice for bridal and engagement contexts.

Selecting and Evaluating Half-Moon Stones

When assessing a half-moon cut — whether as a loose stone or within a mounting — the following points merit particular attention:

• Outline regularity: The curved edge should describe a true, even arc with no flat spots or irregularities. The straight edge must be genuinely straight, not bowed inward or outward.
• Corner angles: The two right-angle corners where the straight and curved edges meet should be clean and precise. Rounded or chipped corners indicate either poor cutting or damage.
• Symmetry of the pair: Hold both stones table-down against a flat white surface and compare outlines directly. Rotate one stone to confirm it is a true mirror image, not merely similar in general shape.
• Windowing: View the stone face-up over a printed page or a dark background. Any legible text or distinct background colour visible through the centre of the stone indicates windowing and shallow pavilion angles.
• Colour evenness: In coloured stones, check for uneven colour distribution, which can appear as a pale zone near the straight edge if the cutter has placed the colour axis unfavourably.
• Calibration: Confirm that the straight-edge measurement matches the corresponding dimension of the centre stone or the mounting specification. A discrepancy of more than 0.2 mm will typically require re-cutting or a custom bezel.

In the Trade

Half-moon stones are produced in calibrated sizes, with the straight-edge length being the primary measurement cited — for example, a "5 × 2.5 mm half-moon" denotes a stone with a 5 mm straight edge and a depth from straight edge to the apex of the arc of approximately 2.5 mm, though conventions vary slightly between cutters and regions. Matched pairs command a premium over single stones of equivalent total weight, reflecting the additional labour of producing symmetrical outlines from separate pieces of rough.

In the diamond trade, half-moon cuts are graded by the major laboratories — GIA, IGI, and others — under their fancy-shape programmes, with cut quality assessed descriptively rather than against the precise numerical standards applied to round brilliants. For coloured stones, no universal grading standard for half-moon cut quality exists; assessment remains largely a matter of trained visual evaluation. Buyers sourcing half-moons for high-value commissions are advised to request stones with laboratory reports confirming species, treatment status, and, where relevant, geographic origin — particularly for sapphire and spinel, where provenance carries significant market weight.

The alias lunette — from the French for "little moon" — is used more commonly in French and Italian trade contexts and in period auction catalogue descriptions than in contemporary English-language retail. The term half-moon is the standard English designation and the one most likely to be recognised across international markets.