Diamond Cut
Cut | Color | Clarity | Carat Weight
This one of the hardest properties to judge, plus there are a number of factors to consider. The first one has to do with the brilliance of the gem.
The pavilion facets of the gem are intended to act as mirrors, to reflect the light entering the stone, back towards the observer. However, the angle they are cut at has a lot to do with how efficiently they work.
The ideal angle for diamond pavilion facets is 41 degrees. This is usually quite convenient, based on the shape of a standard diamond crystal. Unfortunately, not all mined diamonds are in excellent proportions. The diamond cutter is often faced with having to compromise between maximum brilliance and maximum yield. The economics are such that, if the cutter removes too much material from the original crystal, there is no profit in it. Hence, many diamonds get cut at less than ideal proportions.
Diamonds have a high refractive index, which gives them their great brilliance. A little cheating here or there is insignificant. However, if the cutter varies a little further from the ideal the brilliance begins to suffer. Still more and you get a gem that just doesn't stand up to others in terms of brilliance or fire.
There are no grades for this. Most jewelers are familiar with correct proportions and can judge it from the shape. The best test for most of us is to simply compare the gems side by side. If you have two diamonds of the same grade and one is significantly brighter than the other, the cut is the difference.
Please understand that the above discussion assumes we are talking about round diamonds. Because of their symmetrical proportions, all the major facets can be cut at the same angle. The same does not hold true for other shapes.
Many people prefer a "princess" cut. This is fine, but do not expect a princess, or any other shape, to be as brilliant as a round. On a "princess" cut it is necessary to cut a number of facets to accommodate the shape. The angles of these facets vary, slightly to greatly, from those that give the greatest brilliance. This is a simple fact of physics: the more facets that are cut at the ideal angle, the greater the brilliance of the gem.
When looking for diamonds you may come across the terms, "Single Cut," "Old Mine Cut" or "European Cut." Single cut diamonds have a total of 17 facets. Most other designs have 58 facets.
These "single cuts" are usually used on small accent stones, but occasionally you will find an older diamond of decent size with this cutting. Obviously, these gems won't have the brilliance of a full cut diamond, therefore they aren't worth as much.
Another factor that comes under the heading of cut have to do with the shape of the gem. An ideal cut gem should be symmetrical, not lop sided. This point should be obvious, but sometimes it is helpful to point it out. A misproportioned gem can be camouflaged in it's setting and you might not notice it until you have paid for it. This may not bother you, but it might lead to dissapointment.
Though hard, diamonds are also somewhat brittle. The girdle of the gem is the widest part when viewed from the top and the thinnest when viewed from the side. If cut too thin, it can present a weak area that is just asking for trouble. These illustrations will give you an idea of what normal proportions are. Some girdles get cut to a knife edge and this is definitely something to be avoided.