The angle formed by the bezel facets and the girdle plane

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The separation of the white light into spectral colors

An optical property- the difference between the RI values of specific violet and red wavelengths of visible light for a given material.

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top part of the gem above the girdle

Small facet at the bottom of a finished gem

If no culet AGS: Pointed

GIA: None

The star facets are where the arrow heads come from and the lower girdle halves are where the arrow shafts come from when viewing a Hearts and arrows image. On diamonds with ideal optical symmetry, the optics emitted through the star facets have a direct correlation to lower girdle facet length.  So in that sense the star facets are dependent upon that other measurement. When dealing with Ideal cut diamonds you can have a range for both the star facets and lower halves that will give you great performance, there isn’t one magic number or combination.

In our A CUT ABOVE group we personally allow a range of 48-55% in the star facets and a range of 76-80% in the lower halves as we find these to give best result in overall consistent light return. Though it’s not to say that there can’t be great performing diamonds that fall just outside that range, ultimately it’s going to be the performance images (ASET, Idealscope, Hearts) that show you the stones true performance, so I like to think of the star facet as just a detail of a diamond. 

Graders usually base a fancy cut’s color grade on the amount of color they see in the pavilion in the diagonal direction. That’s because color can be more concentrated in the lengthwise direction and less concentrated at the stone’s width. Variations in cut might cause the color of faint yellow or darker diamonds to weaken when they’re viewed through the pavilion, so graders also check color in the face-up position. If the color looks darker than it did when the stone was face-down, they adjust the grade, but never by more than one full grade.

• The whole is greater than the sum of its parts.” Meaning that the overall look of a fancy cut is often much more important than the specific details that would be vital to a round brilliant.
• 3 different cuts: Brilliant, Step, Mixed

§  -Brilliant: Triangular and kite shaped facets that radiate from a gem’s center toward its girdle.

§  -Step: Long, narrow, four-sided facets arranged in rows that are parallel to the girdle on both the crown and pavilion.

§  -Mixed: Have either brilliant-cut crowns and step-cut pavilions or brilliant-cut pavilions and step-cut crowns

• To be well proportioned, a gem’s pavilion should be about 2.5 to 4.5 times the depth of its crown.

• Bow-Tie: Common in shallow or very deep stones, while slightly deep pavilions  can eliminate it. The bow-tie darkens as the difference between a diamond’s length and width increases and pavilion angle variations become more extreme.
• Pavilion Bulge: Brilliants have mains that run straight from girdle to culet, so pavilion bulge isn’t a factor for them. Step cuts, on the other hand, have concentric rows of pavilion facets, making a certain amount of pavilion bulge inevitable. Cutters will sometimes cut the top row of pavilion facets wide and set them at a very steep angle. This creates a pronounced bulge. It can save weight, but it doesn’t increase the stone’s face-up size. It also increases unplanned light leakage. The stone doesn’t look any bigger but it does add to the carat weight and cost. In a step cut, bulge doesn’t affect a stone’s value unless there’s more bulge than necessary for the stone shape.