An open culet is a small polished facet at the pointed base of a gemstone pavilion, in contrast to a pointed culet (sometimes called a closed culet) where the pavilion facets meet exactly at a single point. The open culet was the standard practice in diamond cutting from the development of the brilliant cut in the seventeenth century until the early twentieth century, when improved cutting techniques and a market preference for the maximum brilliance of a pointed culet shifted standard practice. Open culets remain characteristic of Old European, Old Mine, and other antique cuts, and are also found in some modern fancy-cut and emerald-cut stones where a small culet facet is preferred for technical or aesthetic reasons.

Function and history

The original purpose of the open culet was practical: the pointed tip of a pavilion is the most fragile part of a faceted stone, and a small polished facet at the apex protects the tip from chipping during setting and wear. In the era before laser indexing and computer-controlled bruting, the practical chip-resistance gain from an open culet was substantial, and most cut diamonds and coloured stones from before approximately 1900 show a measurable open culet.

Cut analysts working from photography and shadow profiles can use culet character as one element of attribution: an Old Mine cut typically shows a substantial open culet, often visible as a clearly distinct facet through the table; an Old European cut shows a smaller open culet; modern round brilliants from the late twentieth century onward generally show a pointed culet. The transition from open to pointed culet is gradual across the early-to-mid-twentieth century, and individual cutters and cutting centres made the transition at different times. Antwerp cutters generally moved to pointed culets earlier than the Indian and South African production centres, although the regional variation is considerable.

Visibility and grading

An open culet is visible through the table of the stone as a small dark window at the centre of the pavilion. The size of the window corresponds to the size of the culet facet relative to the stone, and on antique cuts can be a defining visual feature. GIA grading standards classify culet sizes as none (pointed), very small, small, medium, slightly large, large, and very large, with each size description corresponding to a measurable percentage of the table diameter.

For modern round-brilliant grading, any culet larger than 'small' typically affects the cut grade, and pronounced large culets reduce the cut grade meaningfully. The optical reason is that a large open culet acts as a small light leak: light entering the stone can exit through the culet facet rather than returning to the eye through the table, reducing apparent brightness. For antique cuts being graded as antique cuts, the culet is described in the same terms but understood as a feature of the antique style rather than a fault. Buyers and graders should describe the culet accurately so that valuation reflects the cut category correctly.

The transition to pointed culets

The transition from open to pointed culet through the early twentieth century reflects several converging factors. Cutting techniques improved, allowing cutters to produce a sharp pavilion apex without unacceptable risk of chipping during the cut. Setting techniques improved, with better tools and tighter tolerances reducing the risk of damage during setting. The market preference shifted toward maximum brightness, supported by the optical analysis showing that a pointed culet returned more light than an open culet. And the diamond grading standards developed by GIA and predecessors codified the preference for pointed culets in modern round brilliants by penalising large culets in the cut grade.

By the mid-twentieth century, pointed culets were standard for new round-brilliant production from the major cutting centres. Older stones with open culets continued to circulate in the antique and estate market, and some specialty cutters continued to produce open-culet stones for collectors and for restoration work, but new production overwhelmingly moved to pointed culets.

Modern applications

Some modern fancy cuts retain a small open culet as a deliberate design element. Emerald-cut stones, particularly, may show a small culet facet rather than a knife-edge keel, to reduce the risk of chipping along the long pavilion edge. Asscher cuts, with their stepped pavilion construction, also typically show a small culet. Old European cut revivals — modern stones cut to evoke the antique style — necessarily include open culets as part of the period idiom, and some buyers specifically seek modern Old European cuts for the antique character without the antique-stone wear and chip patterns.

Coloured stones also use open culets more often than modern round-brilliant diamonds, partly because of the chip-resistance argument and partly because the colour and pleochroism of coloured stones are sometimes better presented with a slightly different optical configuration than the perfectly pointed-culet diamond. Emerald cuts in emerald, sapphire, and other coloured stones routinely show open culets.

Identification and inspection

Identifying an open culet is straightforward. Viewing the stone face-up through a loupe at 10x magnification reveals the culet through the table; an open culet appears as a polished facet at the centre, while a pointed culet shows the meeting point of the pavilion mains as a single fine line or apex. The size of the open culet can be estimated visually or measured against the table size.

For more precise measurement, GIA and other laboratory standards specify culet size as a percentage of the average girdle diameter, calculated from the linear measurement of the culet facet. Modern grading reports include culet description as part of the proportions section.

In the trade

For working jewellers and dealers, accurate description of culet character matters in cut attribution and in valuation. An Old European cut diamond with an appropriately small open culet sells differently from a modern round brilliant with the same weight and clarity, and the open culet is part of what defines the antique-cut value proposition. Buyers seeking specifically antique cuts should expect open culets as part of the package; buyers seeking maximum modern brilliance should select pointed-culet stones. Restoration and reset work should preserve the original culet character where possible, since recutting an antique stone to remove the open culet damages the antique-cut authenticity and reduces value.

See also pointed culet, Old European cut, Old Mine cut, and cut grading for related entries.

Further reading

• GIA Gems & Gemology — diamond cut and grading articles
• GIA — diamond cut grading reference
• International Gem Society — diamond cut and culet reference