Importance of Diameter In Buying Loose Diamonds

Thu, 03/28/2013 - 09:34

When buying a loose diamond, carat weight has a position of preeminence, not just as one of the classic "4 C's", but also as a primary determinant of price. This is true in spite of the fact that a diamond's diameter is a far more accurate determinant of apparent size.

Carat weight refers to the actual weight of a diamond. A carat is equal to .2 grams (roughly the weight of a paper clip), and has no relationship with "karat", the measure of gold's purity. If you consider the weight of a paper clip in your hand (it really does not register as any weight at all), you can see the folly in using carat weight as the primary indicator of diamond's size. Regardless, people the world over refer to diamond size almost exclusively in carats, evaluating the sale price of loose diamonds by whether it is over or under some arbitrary weight such as 1 carat, or 2 carats.

What is really being referred to is not the diamond's weight, which is nearly undetectable on the finger, but to it's size. Size and weight are two different things, although they correlate. The size of a diamond is measured in millimeters, using three measurements (the length, width, and depth). Length and width refer to measurements across the top of the diamond. In round diamonds, these two measurements will be very close since a circle has one diameter all the way round. In fancy shapes, the two measurement will diverge from each other depending on how elongated the shape is. The third measurement (depth) refers to the distance from the top of the diamond surface to the bottom point (or culet). The third measure is important for evaluating cut in a diamond, but is not a good indicator of a diamond's apparent size, since the depth is usually hidden in the ring setting.

Where the distinction between size and weight is most important is when a purchaser is evaluating several seemingly similar diamonds, trying to decide which is best. For instance, two round diamonds, both of Excellent cut, G color, VS2 clarity, both the same price per carat. If one diamond is 1.00 ct., and the other 1.05 ct, the second will be 5% more expensive. However, it is very possible that the two diamonds are of identical diameter. They both might be exactly 6.50 mm across the face, but the 1.05 ct. is slightly deeper in depth, which would account for the extra weight. So, the customer would pay 5% more for a diamond that is exactly the same size (once set) as the 1.00 ct. option. This is true for all diamond shapes.

The best approach is to use carat weight as a general guide in narrowing your search for a loose diamond. You might decide to limit your search to diamonds between 1.50-1.75 carats. However, once you begin ranking diamonds, or choosing a diamond to purchase, you would want to compare them to each other using the dimensions rather than simply carat weight. This is especially true since the price per carat of a diamond is influenced by its size, not just weight. Meaning, if you are looking at several very similar diamonds, the diamond with the lowest price per carat is often the diamond with the smallest length and width measurements.

Lumera provides the measurements as well as carat weight for every diamond, so that size can be evaluated accurately. If you are considering a purchase from Lumera, and have questions regarding the relationship between size and weight, please email or call a consultant for guidance. We would be happy to help.