Carat

This, of course, is the first thing people think of and notice about diamonds, especially engagement rings. "How big is it?" is often the phrase that follows "Congratulations!" when you hear that someone is engaged. However, "bigger is better" is not always the best advice when selecting a diamond.

The Optical Illusion

The Optical Illusion relates to how a stone is cut as well as its carat weight. A diamond can actually be cut to look bigger than it is. For example, a properly cut round brilliant diamond that weights about one carat should be approximately 6.5 mm across. However, a stone weighing much less than that could be cut too shallow to make it appear as large as the properly-proportioned stone. Some jewelers may try to put a higher price on this stone, because it appears to be larger than it really is.

The opposite can also occur when a stone is cut too deeply, but weighs the same as a properly cut stone. In both cases, the poorly cut stones cost the jeweler less, but are sometimes not discounted to the public accordingly.

The Optical Illusion is one very good reason to always buy certified stones graded by independent laboratories. The stone is evaluated by a third party that is not financially invested in the quality or size of the stone. This means you get an unbiased evaluation.

Diamond Math

Everyone knows that a bigger diamond is more expensive, but the price increases are not that straight-forward. For example, a one carat diamond should cost about twce as much as a similar half-carat diamond, right? Not true. The price of a diamond increases exponentially as the size goes up. So, if the half-carat stone costs $5, then a one carat stone would cost $15 and a two carat stone would cost $50.

Big Points and Big Leaps

There are certain points as a diamond's size increases where the price does even more gymnastics. For example, going up two to four points doesn't usually have a great impact on the price of a diamond. However, if that increase crosses any of the "big points," the price will take a "big leap."