If you're someone that owns some beautiful diamonds or are trying to pick one out for a special someone, you might not really care that much about how they were actually cut.  Sure, you might recognize the different shapes they wind up being and may find your eye drawn to one particular shape over another, but if you really understood the science and the art that goes into cutting diamonds, you might yourself with an entirely new appreciation for those beautiful stones.

In gemology, you don't want to confuse the cut of diamonds with their shape.  This is a common mistake.  The shape of the stones refers to its outward appearance and whether or not it's round, pear-shaped, square, and so on.  However, cut refers to the internal qualities of diamonds, and is probably the most important of the Four Cs of diamonds.

A good cut is what gives the stone its brilliance; the angles and finishes of the cut are what give the stones their abilities to handle and reflect light, which is what makes the gem sparkle the way it does.  A fine cut means that light enters into the gem and reflects from side to side before reflecting back out; this is what makes diamonds appear to "flash" when you hold them in the light.  A deep cut will mean that the light will be absorbed into the stone and not reflect much before traveling out of it; while this means that the stone might have a higher carat or weight, it will not be as brilliant and sparkly as one that has an ideal cut.  Sometimes jewelers will have diamonds of a higher carat but poor cut at a discounted price; many naive buyers don't understand that the weight of the stone is not what determines its value alone and are taken in by the thought of a larger stone at a lower price.

The cut of diamonds is classified as Ideal, Premium, Very Good, Good, and Fair or Poor.  Obviously a stone with an Ideal cut may be smaller than another stone but will actually look much better because of how it reflects the light.  At the same time, some diamonds have a higher carat but only a Good cut; they may not be as sparkly and fiery as other stones but are simply larger.

The proportions of the stone are part of what determines how they are cut.  Remember that just like any other stones you see in nature, no two diamonds are alike.  An experienced cutter can work with an uncut stone to bring out as much brilliance as possible by working the edges, angles and finishes, but he or she can only do so much with the stone as it is.  And as a buyer, you need to determine for yourself which is more important - a larger but duller rock or one that is smaller but technically more valuable.  Once you understand how diamonds are cut you can then make the best decision for yourself.