Diamond carat explained

Often people think that "carat" refers to a diamond’s size, and certainly the bigger the carat the larger the diamond. But it is actually the weight of the gem that we measure in carat. The name carat is derived from the carob seed, which has been used since ancient times to measure a gem’s weight.


One carat is equal to 200 milligrams, or 0.2 of a gram. A diamond's weight is usually indicated in carats, but you may also hear it mentioned as points when a diamond is smaller than 1 carat. For example a 0.30 carat diamond, may also be referred to as 30 points or a 30-pointer. 

Why is carat weight important?

As diamonds come in different shapes the total weight of a gem is the most objective way to measure its size. Gems are valued in price bands by carat weight. The larger the diamond, the rarer it is. This is reflected in the price per carat: often there will be a marked price jump in the price per carat at a round number like the popular 1 carat mark, again at 1.50 carats, and so on.

The jewellery industry standard for pricing diamonds is the The Rapaport Diamond Report. Jewellers use it as a guideline when determining diamond prices.

If you'd like to take a closer look, the chart above show the difference in price between round diamonds of 0.90-0.99 carats (left) and of 1.00-1.49 carat (right), comparing colour and clarity. For example a 0.95ct diamond of F colour and VS2 colour has a per carat price of $7800, whereas a larger 1.05 ct jumps to $11500 per carat. 

Before adoption of the standardised “ideal cut”, gems were cut to retain the maximum carat weight of the finished gem. This sometimes leads to gems with unnecessarily heavy proportions. For example old mine cuts are often nearly cushion shaped (rather than round), with taller tables and deeper or fatter pavilions potentially with a large flat culet - all adding up to a higher carat weight. They may not be up to modern ideal standards, but these gems have gorgeous character all of their own. Here are a few striking examples in our collection.

As diamonds became more readily available they became a status symbol. In certain circles size was valued over other factors and some diamonds were cut to look as big as possible. For example a 1 carat round brilliant diamond might be cut with a spread (or diameter) of a 1.2 carat diamond. Unfortunately to compensate for this, the crown and pavilion must be cut so shallow that they lack brilliance and scintillation, detracting from the gem’s beauty. Yes, sometimes bigger is not better. Sorry, we have no examples of these in our collection!

So what sizes are we talking about?

Now that the ideal cut is so widely accepted and gems are increasingly precision cut to these standards, most diamonds of a particular carat weight will be the same size (within a small margin).

One carat is a popular choice for a diamond engagement ring and a well-cut 1 carat diamond will measure 6.5mm across. For reference, a well-proportioned 2-carat diamond has a diameter of 8.2mm and a 3-carat gem will measure 9.4mm across. Following the specifications of the ideal cut, the larger gems will not only be wider, but also taller making for a big jump in wow factor.

Tips for selecting your diamond

If you’re looking for the biggest gem for your budget, we recommend sticking with the correct proportions (ideal cut) and maybe sacrificing a little on one of the other C’s. For example a carefully selected H-I colour diamond with SI1 clarity is a gorgeous gem at a fraction of the cost of a D-F VVS diamond.

If your love is for big shiny diamonds, consider adding a halo of smaller diamonds to your centre gem - the total spread of the cluster will look like a bigger diamond. Or set the gem in a bezel to trick the eye into seeing a larger gem. A white gold or platinum setting will also make your diamond look bigger.

The Cannele below has a 0.33 carat diamond at the centre, but the halo, platinum setting and slim shank, make it look like a 1 carat diamond. In the next ring, the  0.35 carat solitaire in a white gold split-bezel setting, tricks the eye to see a handsome 0.80 carat diamond.

Another tip is to get a diamond that falls just short of the full carat mark. As we mentioned discussed earlier the larger the diamond, the rarer, and thus more expensive it is. There is a marked jump in the price per carat at a round number like the popular 1 carat mark. Buying a diamond that sits just under the whole carat, for example a 0.98 carat diamond, will be visually the same size, but the price per carat will be significantly lower.

Something else to take into account when choosing a diamond is the size of the hand that will wear it. A one carat diamond will appear larger on a small hand with slim fingers, or comparatively smaller on a larger hand with wider fingers.

 
Also it is well worth your effort to do some research into lab grown diamonds - they are every bit as beautiful as their natural diamond and no one will be able to tell the difference (but your pocket will!)

Want to learn more? Book your appointment to view diamonds at DesignYard.