The Round Diamond Cut
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The diamond cut often refers to two separate characteristics of a loose diamond: (1) its shape, and (2) its make, or style of cutting and quality, which consists of polish and symmetry.
Polish and Symmetry
A diamond's shape as well as its polish and symmetry affect the "behavior" of the stone—simply speaking, how it reflects light. In general, a diamond's cut is all about maximizing the optical light effects that determine the ultimate beauty of the diamond: brilliance (amount of light reflected back to viewer), fire (split of light into a rainbow's colors), and scintillation (glittering of reflected light in the crystal and visibility of dark spots when stone is moved).
Polish and symmetry significantly impact all three of these. In the following picture you can see the most critical parts of a diamond and their names.
To ensure that the diamond has the best brilliance and fire, the best option is to stay away from stones with proportions below the ideal standard. Below you can find the explanation for all facets of a diamond and standards below which you should not buy the stone. The information about such standards can be found in the diamond certificate. GIA calls their certificates diamond dossiers or diamond grading reports while AGS calls theirs diamond quality documents or diamond quality reports.
A diamond's most critical parts:
- Diameter of a loose diamond, measured at the Girdle, which is the widest portion of a stone.
- Girdle – the middle portion of a diamond stone, its widest part. The Girdle is measured from "extremely thin" to "extremely thick" – the "medium" thickness Girdle is preferred (you can find the information about it in the diamond certificate). Stay away from a "thin" Girdle as it is too fragile and can chip more easily, also do not buy a "thick" one either as it is unnecessarily adding weight to the stone where it matters the least.
- Table – the top of the diamond, whose area size ideally should be 53% to 57.5% (in a Round Brilliant) of the Girdle diameter according to the American Gemological Society (AGS) lab studies. Some jewelers say up to 64% is still acceptable. Run away from the diamond with the Table area above 64%.
- Depth – in a Round Brilliant, length of the stone from the surface of the Table to the bottom of its Pavilion). Depth should ideally be 58% to 63% of the stone Diameter
- Crown – the portion of the diamond between its girdle and its table) – the Crown angle in the well-cut diamond should be 33 to 35 degrees.
- Pavilion – in a Round Brilliant, the cone-shaped lower part of the stone:
a. The Pavilion Depth of the stone, according to the AGS Lab, should ideally be 42.5% to 43.5%. Pavilion Depth is a portion of the overall diamond Depth and represents the height of a diamond Pavilion or the distance from the Girdle to the bottom of Pavilion, called Culet. In diamonds with very deep Pavilions, the entire surface of the Table appears to be dark creating what industry experts call a "Nailhead". On the other hand, stones with shallow Pavilions often produce a "Fisheye" effect due to Girdle's reflection in the diamond's Table. So, if the Certificate specifies the Pavilion Depth is above 43.5% or below 42.5% – the diamond's sparkle will be diminished.
b. Pavilion angle is another important dimension of the stone, which determines its brilliance and fire. However, many GIA certificates (called Diamond Dossiers and Reports) do not provide information on the diamond's Pavilion angle. So, you will have to rely on the information on the stone's overall quality of the Polish and Symmetry as well its Table size and Depth (all GIA Dossiers and Reports provide this information as a bare minimum).
c. The relationship between the Crown angle and the Pavilion angle (see above) has the greatest effect on the look of the diamond. A slightly steep Pavilion angle can be complemented by a shallower Crown angle, and vice versa.
- Culet – the facet at the bottom tip of a gemstone. The quality of the Culet is typically specified in a diamond certificate – the preferred Culet is not visible with the unaided eye (so, the best one should be graded either "None" or "Medium").
Ratios and proportions of various diamond dimensions in relation to each other are what impact the brilliance and scintillation of the stone most significantly. They determine how well (or not so well) and how much of the light is reflected inside the crystal and back to the surface through the diamond's table. You can find this information on certificate specification enclosed with every certified loose diamond. If the diamond is not certified by one of the gemological labs, then buying such a diamond is not advisable, unless you are a professional gemologist and can certify diamond specifications yourself.
The GIA (the Gemological Institute of America) grades diamonds with descriptive words Excellent, Very Good, Good, Fair, and Poor to convey the quality of the gemstones' Cuts.
The 'polish' grade describes the smoothness of the diamond's facets. The symmetry grade refers to alignment of the facets in relation to each other:
- Poor polish of the diamond surface can dull the light radiating from the stone. It may also create blurred or dulled sparkle.
- Poor symmetry can misdirect the light inside the stone as it enters and exits the diamond – the more light is lost due to poor symmetry, the less sparkly the stone would look and the more dark spots you can observe when looking at the diamond from the top.