“I don’t want anything too big, maybe just a carat or two,” Olivia said to the salesman, her soon-to-be fiancé nodding in agreement. The couple, in their mid-twenties, had only just started shopping for an engagement ring. The salesman pulled out a classic solitaire ring within their budget, explaining the center diamond was 1.52 carats, and when Olivia slipped the ring onto her finger, the diamond was much bigger than she expected (not that she minded)! She asked to see another ring – this one was described as being 1.60 carats – and when she put them next to each other to compare, she noticed the second stone was smaller than the first she’d tried on. How is that possible? she thought. They must have switched the tags by accident.
The salesman hadn’t accidentally switched the tags. In fact, the diamonds were inscribed with numbers that had also been printed onto their tags – meaning a quick glance in the microscope confirmed the salesman was correct.
Carat weight is the most apparent of the four diamond quality factors (called the “4 Cs”) to most customers, and for good reason – the size of the diamond is by far the easiest to see! But in truth, carat weight is just that: a measurement of the stone’s weight. One carat is equivalent to one-fifth of a gram, and is measured on a very sensitive scale, usually to the thousandth or even ten-thousandth of a carat. The carat weight is then presented with two decimal places, called “points”; a 0.70 carat diamond can also be called a seventy point diamond. It’s easiest to think of carats like dollars, and points like cents.
Normally, the weight of a diamond does correlate with size. A one-carat round diamond can be estimated to have a diameter of 6.5mm on average, which, compared to the average ring finger width of 15mm, is bigger than many expect! When a diamond cannot be weighed, basic “carat to millimeter” conversion charts are often used to estimate the carat weight based on the diameter of a particular diamond, or vice versa. But as shown in Olivia’s case, these charts aren’t always reliable. Sometimes, a diamond can have a larger or smaller diameter than its carat weight would indicate. The reason is simple: diamonds are three-dimensional!
The depth of a diamond can have a drastic effect on its carat weight without affecting its apparent size. Picture a cube. If you take a slice off the bottom, it’ll still look the same from above, but weigh less; similarly, if you add a section to the bottom, it’ll weigh more without changing its appearance from the top view. Increasing or decreasing the “bulk” around the girdle (or edge) of the diamond will change its weight in the same way, as will changing the angles of the top and bottom (crown and pavilion respectively). This means the most accurate way to estimate carat weight involves measuring the depth of the diamond, and using this measurement along with the average diameter to calculate what the weight should be. This method is usually fairly accurate, but, of course, simply weighing the diamond will always be the best way!
So how should Olivia choose between the two diamonds presented to her? All other factors being equal, she should simply choose the size which she prefers on her finger. But such a large difference in carat weight usually indicates one or both of the diamonds has a lower cut grade – a different topic for a different post – so in that case, Olivia might look at the diamonds side-by-side, and decide which looks prettier to her. A very shallow diamond, called “spready” by many people in the jewelry trade, will look larger than its weight may indicate, but could appear grey or lack fire. A deep diamond will look smaller than its weight indicates, and may be dark and lack brilliance. The first diamond OIivia looked at, weighing 1.52 carats, was one of the so-called “spready” stones, and so she ended up preferring the heavier, smaller-looking diamond – but another customer, who wanted a larger-looking stone on a tight budget, found that the shallower diamond suited their needs perfectly!