Oval Cabochon
Oval Cabochon
The standard cabochon outline for phenomenal and opaque gem materials
The oval cabochon is the most widely used cabochon shape in commercial coloured-stone production, a polished, undomed-on-bottom, domed-on-top form with an oval outline that suits both the geometry of most rough and the optical phenomena that the cabochon cut is designed to display. The shape is the standard for phenomenal materials such as star sapphire and ruby, cat's-eye chrysoberyl, and moonstone, and is equally common in opaque and translucent materials including jadeite, opal, turquoise, lapis lazuli, and chalcedony.
Geometry and proportion
An oval cabochon has a flat or slightly curved base and a domed top. The outline is a symmetrical oval with both ends equally rounded; the dome rises from the girdle line, typically reaching its maximum height directly above the centre of the outline. Standard length-to-width proportions in the trade run from approximately 1.25 to 1 through to 1.75 to 1, with 1.4 to 1 a common compromise that pairs well with most ring and pendant settings.
Dome height is the variable that most affects appearance. A high dome is essential to the centring of asterism and chatoyancy and to the depth of phenomenon in fine moonstone, while a low dome is preferred where the material is being cut for pattern or colour rather than for an optical effect. Standard dome ratios in the trade run from approximately one-quarter of the short-axis width for low-dome work to two-thirds or more for high-dome phenomenal cuts.
Material applications
Star corundum is invariably cut as an oval or round cabochon, with the c-axis of the crystal oriented perpendicular to the base of the cabochon so that the rutile silk needles produce a centred six-rayed star at the apex of the dome. Cat's-eye chrysoberyl and other chatoyant stones are cut with the parallel inclusions oriented across the long axis of the oval so that the eye runs the short way. Moonstone is cut with the lamellar plane parallel to the base, and the dome is set high to maximise the floating sheen.
For opaque ornamental materials, the oval cabochon is the standard yield-maximising shape for the typical rounded rough. Jadeite, turquoise, lapis, and opal are routinely cut as ovals, with the cutter selecting outline ratio and dome height to optimise yield and to centre the most desirable colour or pattern at the apex of the dome.
Yield and cutting
The oval cabochon yields more from typical rough than the round cabochon and is therefore the default commercial choice. The cutter shapes the rough on a coarse wheel, refines the outline on a finer wheel, and polishes through progressively finer stages of diamond, alumina, or cerium oxide compound. Symmetry of the outline and the smooth transition from girdle to dome are the principal quality markers; a poorly executed oval cabochon will show flat spots, asymmetry, or a girdle that is not parallel to the base.
Settings and durability
The oval cabochon is well suited to bezel settings, which protect the girdle and provide a clean visual frame. Prong settings are also workable for harder materials. The flat or near-flat base of the cabochon allows for a low setting profile, useful in ring designs intended for daily wear. The shape is comfortable on the finger and translates well across pendant, earring, and brooch designs.
In the trade
Oval cabochon is the lingua franca of the coloured-stone cabochon market. Sizes are quoted in millimetres along the long and short axes — for example 10 by 8 millimetre oval cabochon is a standard commercial calibre — rather than in carat weight, because cabochon weight depends so heavily on dome height. Standard millimetre calibres are used in production jewellery to match findings and pre-made mountings.