Total carat weight (TCW)

A carat is a unit of weight, 200 milligrams, divided into 100 points. Carat weight on its own describes one stone. Total carat weight adds up the weights of all the diamonds in a piece, so it is the number to read on rings with side stones, halos, three-stone rings, eternity bands, bridal sets and tennis bracelets.

By convention it counts the diamonds only; the weight of the metal is not part of it. On a ring with one diamond, TCW and the stone's carat weight are the same number. On anything with several stones they are not, and that gap is where listings get misread.

Because the weight is shared among the stones. A three-stone ring of 1.00 ctw might have a 0.50 ct centre and two 0.25 ct side stones. A halo ring of 1.00 ctw might have a 0.70 ct centre and 0.30 ct of small stones around it. Only the first stone in each is large enough to see as the "main" diamond.

Size grows more slowly than weight. For stones cut to similar proportions, diameter rises with the cube root of weight, so by that arithmetic a 0.50 ct round diamond is about 79 per cent as wide as a 1.00 ct one, and a 0.33 ct stone about 69 per cent. A centre stone of half the headline weight is therefore not half the width, but it is clearly smaller than the single 1 ct solitaire it might be mistaken for.

Rarity follows the same logic. GIA notes that larger diamonds are rarer, so a single large stone is valued differently from the same total weight spread over many small ones. That is why a piece of 1 ctw made of many small stones is not comparable with one 1 ct diamond.

Look for four numbers, and ask for any that are missing:

1. The centre stone's weight, separately from the total.
2. The number of stones and the size of the side stones.
3. The quality of each group. A grading report describes an individual stone, so ask whether the side stones were graded one by one or described as a group for colour and clarity.
4. The stone type. Moissanite is lighter than diamond for its size, so stones of the same weight differ in width. Compare millimetre dimensions as well.

Two listings that both say "1 ctw" can be very different: one with a single 1 ct diamond, one with a 0.40 ct centre and a lot of small stones. Compare the centre stone first, because it is what you will see most.

It should be accurate to the last digit shown. The FTC's Jewelry Guides, which are US guidance, say a weight stated in decimals should be accurate to its last decimal place, so a figure of ".5 carat" stands for a weight from 0.495 to 0.504 ct. If a weight is shown as a fraction, such as 1/2 ctw, the seller should say clearly that it is not exact and give the range or tolerance used. The word "point" should only appear with the decimal carat weight too.

So a listing that says only "1/2 ctw" with no range is leaving out information. Ask what range it covers, or ask for the actual weight to two decimals.

When you plan a custom ring, decide first whether the centre stone or the overall look matters more, then take that to the design studio or a showroom. The custom engagement rings service starts from the shape and setting you choose.