The cut of a brilliant diamond may be the most singular important consideration in buying a stone within a set price range. Unless one is an expert and feels his knowledge is good enough to override general public consideration, there is only one cut to consider and that is the “brilliant” cut. Brilliant cut is a modern cut that is a completely round stone designed with 58 facets to maximize light reflection and “fire” within a diamond.
There are a lot of stones still around which have what is known as a European cut. This cut was done in the 1920’s and before and does not compare in value to the modern brilliant cut. The old cut or European cut stones were cut before exact ratios and angles were established and understood by the gem cutting society and, as such, do not maximize the reflecting and refraction qualities of the stone. European cut stones such as those purchased at pawn shops and estate sales, are much harder to resell and do not offer the liquidity of a brilliant cut diamond.
There are other popular modern cuts such as the marquise, the oval and the pear which attract some buyers when designed for jewelry, due to their unique appearance. Fancy cut diamonds have fewer angles cut to what is known as the “critical angle” and, as such, cannot be as brilliant as a round cut stone. The fancier a stone is, the more it differs from a brilliant cut, the greater the loss in light reflection will be.
Fancy cut stones have only two bottom facets as opposed to the eight found in round cut stones to reflect the light back. While they still may appear to be fairly brilliant, the refraction, the fire of the stone, will suffer critically. This loss progresses from the marquise cut through the straight cuts such as the emerald cut diamond. These straight cut stones suffer a great light and fire loss, but still looks magnificent regardless.
An uncut diamond is normally sawn or split into two or more stones as decided by the diamond cutter. It just takes a simple error here to completely ruin a valuable stone and turn it into nothing but dust. Once a rough diamond is split, the diamond cutter then decides how the stone will be laid out and cut. This operation means that a certain portion of the diamond will be ground off and lost and so this cut plan becomes an important step in finishing the final stone.
The first step taken by the diamond cutter is to girdle the diamond. This process of girdling establishes the size of the stone and puts a “waste” on the stone. If a stone is poorly girdled, it will not appear completely round when viewed with the jeweler’s loupe or microscope. A round brilliant cut stone should be perfectly round and symmetrical. Other mistakes in girdling will produce flaws that manifest themselves as a razor thin girdle which is prone to chipping or breaking (even though diamonds are extremely hard, they are brittle and can be chipped or shattered in thin areas). A too thick girdle takes away from the brilliance and fire of a stone and indicates a poor job on the part of the diamond cutter.
A diamond cutter cuts (in a brilliant cut) 58 facets all done on exact angles in exact positions in order to let the diamond reflect as much light as is physically possible. The brilliant cut stone has 16 facets on top and 16 facets on the bottom that reflect the light and give the stone its cut. Each facet is cut on a unique angle and is exactly straight when viewed with other facets in order to maximize light reflection. When you view a brilliant cut stone, around the table of the stone you’ll see the kite and the topmain facets. These facets are the areas that allow the light to come through to the viewer. Beneath these you have eight star facets and then 16 upper girdle facets before you reach the girdle itself. Beneath the girdle you have an additional 16 lower girdle facets. All these ancillary facets contribute to the light reflection through the kite and top main facets and the table portion of the stone.
What is the advantage of the 58 facet brilliant cut stone? What does one expect to see when viewing a diamond? There are two qualities that make a diamond attractive to the eye. The first one is known as life and indicates the amount of light that is reflected back from the diamond to the viewer. The second quality is known as fire, which is an indication of the amount of refraction from the facets and split into colors as in a prism effect.
Besides the 58 facets, a number of other factors contribute to the perfectness of a brilliant cut stone. The stone’s table should be 53% of the area of the stone. While the ratio between the depth of the stone or the length of the stone if you view it from the side, to the spread of the stone which is the maximum diameter of the girdle, this ratio should be 60% depth to spread. The angles on a stone must be cut exactly to critical angles. Any deviation will produce a less than perfect reflection of the light waves entering the stone. A jeweler will have special gauges to measure these angles.
When angles are viewed through this loupe, they can be accurately measured. The first measurement to take is the degree of the angle from the table to the girdle of the stone. This is known as the top critical angle and should be 34 1/2 degrees. Underneath the girdle, the bottom angle from the girdle to the point of the stone is also a critical angle and should be cut at 40 3/4 degrees. A further measurement is that the girdle should be about 1% as thick as the diameter of the stone, although this is not quite as critical as the other measurements and can be judged by the eye after a bit of practice. A stone which is not cut with the critical angles in the right degree, will either be shallow cut or deep cut and will not reflect the light back through the center of the stone (the table of the stone) with the same brilliance as a stone that is cut to the correct angles.
If the stone is shallow cut, the light will reflect off the edges of the stone but not through the middle. If it is cut too deeply, the center of the stone will appear to be dark and it is called “heavy.” In the past some cutters cut the upper angles at a less than 30 degree cut. This “spread cut” helps hide deficiencies in a stone but makes the girdle angles sharp and likely to be broken or chipped and the stone is not as valuable as a normally cut stone.