Editing Diamond cut (section)

== Round brilliant ==
Developed c. 1900, the round [[brilliant (diamond cut)|brilliant]] is the most popular cut given to diamond. It is usually the best choice in terms of saleability, [[insurance|insurability]] (due to its relatively "safe" shape), and desired optics.

=== Facet count and names ===
[[File:diamond facets.svg|right|thumb|Diamond proportions and facets, for the round brilliant cut.]]
The modern round brilliant (Figure 1 and 2) consists of 58 facets (or 57 if the [[culet]] is excluded); 33 on the crown (the top half above the middle or girdle of the stone) and 25 on the pavilion (the lower half below the girdle). The girdle may be frosted, polished smooth, or faceted. In recent decades, most girdles are faceted; many have 32, 64, 80, or 96 facets; these facets are excluded from the total facet count. Likewise, some diamonds may have small extra facets on the crown or pavilion that were created to remove surface imperfections during the [[diamond cutting]] process. Depending on their size and location, they may hurt the [[symmetry]] of the cut and are therefore considered during [[#Cut grading|cut grading]].

Figure 1 assumes that the "thick part of the girdle" is the same thickness at all 16 "thick parts". It does not consider the effects of indexed upper girdle facets.<ref>{{cite web|url=http://www.pricescope.com/journal/effects-indexed-upper-half-facets|title=The Effects of Indexed Upper Half Facets|author=Peter Yantzer, American Gem Society Laboratories|date=2005-03-18}}</ref> Figure 2 is adapted from the Tolkowsky book,<ref>Marcel Tolkowsky (1919) ''Diamond Design'', [http://www.folds.net/diamond_design/index.html#fig_37 Figure 37]. Spon & Chamberlain. Folds.net.</ref> originally published in 1919. Since 1919, the lower girdle facets have become longer. As a result, the pavilion main facets have become narrower.

=== Proportions ===
{{More citations needed|date=January 2007}}
While the facet count is standard, the actual proportions—crown height and crown angle, pavilion depth and pavilion angle, and table size—are not universally agreed upon. There are at least six "ideal cuts" that have been devised over the years, but only three are in common use as a means of benchmarking. Developed by [[Marcel Tolkowsky]] in 1919, the American Standard (also known as the American Ideal and Tolkowsky Brilliant) is the benchmark in North America. It was derived from mathematical calculations that considered both brilliance and fire. The benchmark in Germany and other European countries is the Practical Fine Cut ({{langx|de|Feinschliff der Praxis}}, also known as the Eppler Cut), introduced in 1939. It was developed in Germany by empirical observations and differs only slightly from the American Standard. Introduced as part of the [[Scandinavian Diamond Nomenclature]] (Scan. D. N.) in 1969, the Scandinavian Standard also differs slightly.

Other benchmarks include: the Ideal Brilliant (developed in 1929 by Johnson and Roesch), the Parker Brilliant (1951), and the Eulitz Brilliant (1968)<ref>{{cite journal|title=The Optics of Brilliant-Cut Diamonds|author=W. Eulitz|journal=Gems and Gemology|volume =12 |issue=9 |year=1968 |pages= 263–271}}</ref> The Ideal and Parker brilliants are disused because their proportions result in (by contemporary standards) an unacceptably low brilliance. The Eulitz cut is the only other mathematically derived benchmark; it is also historically the only benchmark to consider girdle thickness. A more modern benchmark is that set by Accredited Gem Appraisers (AGA). Although their standard generally makes a modern ideal cut it has been criticised for being overly strict. A summary of the different benchmarks is given below:

{| class="wikitable"
|-
!Benchmark
!Crown<br />height
!Pavilion<br />depth
!Table<br />diameter
!Girdle<br />thickness
!Crown<br />angle
!Pavilion<br />angle
!Brilliance<br />grade
|-
|AGA
|14.0–16.3%
|42.8–43.2%
|53–59%
|{{n/a}}
|34.0–34.7°
|{{n/a}}
|100%
|-
|American Standard
|16.2%
|43.1%
|53.0%
|{{n/a}}
|34.5°
|40.75°
|99.5%
|-
|Eulitz Brilliant
|14.45%
|43.15%
|56.5%
|1.5%
|33.6°
|40.8°
|100%
|-
|Ideal Brilliant
|19.2%
|40.0%
|56.1%
|{{n/a}}
|41.1°
|38.7°
|98.4%
|-
|Parker Brilliant
|10.5%
|43.4%
|55.9%
|{{n/a}}
|25.5°
|40.9°
|Low
|-
|Practical Fine Cut
|14.4%
|43.2%
|56.0%
|{{n/a}}
|33.2°
|40.8°
|99.95%
|-
|Scandinavian Standard
|14.6%
|43.1%
|57.5%
|{{n/a}}
|34.5°
|40.75°
|99.5%
|}

Crown height, pavilion depth, and table diameter are percentages of the total girdle diameter. Because the pavilion angle (and consequently pavilion depth) is so closely tied to total internal reflection, it varies the least between the different standards.

=== Hearts and arrows phenomenon ===
[[File:Diamond displaying "arrows" - 509861FDFD.jpg|thumb|upright|Diamond displaying eight arrows]]
The term "hearts and arrows" is used to describe the visual effect achieved in a round brilliant cut diamond with perfect symmetry and angles that exhibit a crisp and complete pattern of hearts and arrows.<ref>{{Cite web |title=Ultimate Guide to Hearts and Arrows Diamonds {{!}} Whiteflash |url=https://www.whiteflash.com/diamond-education/benefits-of-hearts-and-arrows-diamonds/ |access-date=2025-04-29 |website=www.whiteflash.com |language=en-us}}</ref> When viewed under a special magnifying viewer, a complete and precise visual pattern of eight hearts is seen while looking down through the pavilion, and eight arrows can be seen when viewing the stone in the table up position.<ref>{{cite web |last1=Rice |first1=Addison |title=Hearts and Arrows Diamond: Is it Worth it? |url=https://www.gemsociety.org/article/hearts-and-arrows-diamond/ |website=www.gemsociety.org |publisher=International Gem Society LLC |access-date=10 January 2025}}</ref>

=== Passion cut ===
Another modification of the round ideal cut that maintains the basic proportions of its angles is the passion cut.<ref>Yariv Har (June 2, 2009). [http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=D593,440.PN.&OS=PN/D593,440&RS=PN/D593,440 "Gemstone with 81 facets"]. United States Patent D593,440.</ref> This cut's design can be considered the opposite of the hearts and arrows, as it eliminates the arrows in order to capture a different light return from the center of the diamond. The cut splits the eight pavilion mains and increases the specifically-placed total facets from 57 to 81. The cut was designed to enhance brilliance and mask inclusions.

[[File:PC DiamondAnatomy B-W.jpg|PC DiamondAnatomy B-W]]