Crown Main Facet
Crown Main Facet
The eight kite-shaped facets that govern a brilliant-cut stone's light return and visual character
A crown main — also called a bezel facet or, in older lapidary nomenclature, a templet — is one of the eight large kite-shaped (rhomboidal) facets that occupy the upper portion of a standard round brilliant-cut diamond or coloured gemstone. Arranged symmetrically around the table, they form the dominant visual plane of the crown, extending from the table edge downward to the girdle. Together with the star facets above them and the upper-girdle facets below, the crown mains constitute the full facet complement of the crown. Their geometry — principally their angle relative to the girdle plane — is among the most consequential variables in cut grading, directly governing brilliance, fire, and scintillation.
Anatomy and Position
In the standard 57-facet round brilliant (58 facets when a culet is included), the crown carries 33 facets in total: one table, eight star facets, eight crown mains, and sixteen upper-girdle (upper-half) facets. The crown mains are the largest individual facets on the crown by surface area. Each is bounded at its upper edge by two sides of the octagonal table, at its lateral edges by two star facets, and at its lower edges by two upper-girdle facets that meet at the girdle. In plan view — looking straight down through the table — the eight crown mains appear as four pairs of opposing kite shapes, their apices pointing inward toward the table corners.
The term bezel facet remains common in trade usage and appears throughout older gemmological literature; the GIA employs both "crown main" and "bezel facet" interchangeably in its grading documentation. The word templet (sometimes spelled template) is largely archaic, surviving mainly in historical lapidary texts and in the terminology of certain European cutting traditions.
Crown Angle: The Critical Parameter
The crown angle is measured as the angle between the plane of a crown main facet and the girdle plane (the horizontal). It is the single most important geometric descriptor of the crown mains and a primary input in any scientific evaluation of cut quality. GIA research, published in Gems & Gemology and underpinning the GIA Cut Grading System for standard round brilliant diamonds, identifies a crown angle range of approximately 32° to 36° as producing the strongest combination of brilliance and fire, with the optimal zone centred near 34° to 35° when paired with a complementary pavilion angle of approximately 40.6° to 41°.
The relationship between crown angle and pavilion angle is not independent: the two interact to determine the path of light rays through the stone. A steeper crown angle (above 36°) tends to increase dispersion (fire) at the expense of some brightness; a shallower crown angle (below 32°) can suppress fire and, in extreme cases, produce a "fisheye" appearance when combined with a shallow pavilion. GIA's cut-grading research demonstrated that no single crown angle value is universally optimal; rather, it is the combination of crown angle, pavilion angle, table size, and girdle thickness that determines overall optical performance.
In practice, crown angle is measured either by contact goniometry on the physical stone or, more commonly today, by optical scanning instruments such as the Sarin DiaMension, OGI systems, or Helium scanners, all of which report crown angle averaged across the eight crown mains. Minor variation among individual mains — ideally less than 1° — is a component of symmetry assessment.
Crown Main Size and Table Percentage
The radial extent of the crown mains is directly linked to table percentage, defined as the diameter of the table expressed as a percentage of the stone's average girdle diameter. A larger table encroaches on the crown mains, reducing their visible surface area; a smaller table enlarges them. Historically, older "Old European" and "Old Mine" cuts featured smaller tables (often 50–55%) and correspondingly large, prominent crown mains that produced strong fire. The shift toward the modern ideal brilliant in the twentieth century brought table percentages into the 54–62% range, balancing fire against brightness. Tables above approximately 65% compress the crown mains to the point where fire is noticeably diminished.
Crown Mains in Fancy Cuts
The term "crown main" applies most precisely to the round brilliant, but the concept extends to all brilliant-style fancy shapes — ovals, pears, marquises, cushions, and hearts — where analogous large crown facets surround the table. In these cuts the crown mains are not always eight in number, and their geometry is adapted to the outline of the stone, but their optical function is identical: to act as primary windows for light entry and exit and as the principal generators of brightness and dispersion. In step cuts such as the emerald cut or Asscher cut, the equivalent facets are rectangular step facets rather than kite-shaped mains, and the term "crown main" is not conventionally applied.
Role in Cut Grading
GIA's cut-grading system for standard round brilliant diamonds, introduced in 2005, incorporates crown angle as one of seven key proportion parameters evaluated during grading. The system uses a predictive model derived from ray-tracing analysis of millions of virtual stones to assign grades of Excellent, Very Good, Good, Fair, or Poor. Crown angle feeds into this model both directly and through its interaction with pavilion angle. An "Excellent" grade requires that the crown angle fall within a range that, in combination with the other parameters, produces predicted brightness, fire, and scintillation scores above defined thresholds.
Beyond GIA, independent cut-analysis systems — including the American Gem Society (AGS) Light Performance grading and the proprietary algorithms used by laboratories such as HRD Antwerp — similarly treat crown angle as a primary variable. The AGS system, which grades on a 0–10 scale, uses ray-tracing to model actual light performance rather than proportion proxies, but crown main geometry remains a central input.
Polishing and Symmetry Considerations
Each crown main must be polished to a flat, mirror surface free of polishing lines, surface graining, or residual roughness. On a grading report, the polish of individual facets is assessed under 10× magnification; polishing lines or abrasions on a crown main are noted as polish characteristics. Symmetry assessment examines whether all eight crown mains are equal in size and shape, whether their apices meet cleanly at the table corners, and whether opposite mains are truly parallel in angle. Deviations — such as a "wavy" table edge caused by unequal crown main heights, or a "table off-centre" caused by asymmetric mains — are recorded as symmetry features and can affect the overall symmetry grade.
Historical Nomenclature
The vocabulary surrounding crown mains reflects centuries of lapidary tradition. The word bezel derives from the Old French bisel, referring to the sloped face of a chisel, and was applied by early diamond cutters to the slanting facets of the crown. Templet appears in seventeenth- and eighteenth-century Dutch and English cutting manuals as a term for the large crown facets of the rose cut and early brilliant forms. As the modern round brilliant became standardised in the early twentieth century — through the work of Marcel Tolkowsky, whose 1919 treatise Diamond Design provided the first mathematical optimisation of brilliant proportions — the terminology gradually consolidated around "bezel facet" in trade usage and "crown main" in more technical gemmological writing.