Gemstone size guide

Carat is a measure of weight, not size. Some gemstones are denser than others, so the same carat weight looks different from one gem to the next: a 1 ct sapphire is a little smaller than a 1 ct diamond, and a 1 ct emerald is noticeably larger. That is why we size our gemstones in millimetres. The carat weights we show are close estimates.

One carat, four gemstones

Diamond
6.5 mm

Sapphire & ruby
6.1 mm

Alexandrite
6.2 mm

Emerald & aquamarine
6.9 mm

Round stones of 1 carat, drawn to scale with each other. Actual size on screen depends on your device.

Round stones: approximate carats by size

Size	Diamond	Sapphire & ruby	Alexandrite	Emerald & aquamarine
4 mm	0.2	0.3	0.3	0.2
5 mm	0.5	0.6	0.5	0.4
6 mm	0.8	1.0	0.9	0.7
7 mm	1.3	1.5	1.4	1.0
8 mm	1.9	2.3	2.1	1.5
9 mm	2.7	3.3	3.0	2.2
10 mm	3.7	4.5	4.2	3.0

Approximate weights for well-proportioned round brilliant stones. Emerald and aquamarine are both beryl, so they weigh almost the same at the same size.

Our shapes at about one carat

Round
6 mm · ~1.0 ct

Oval
7×5 mm · ~0.9 ct

Cushion
6×6 mm · ~1.2 ct

Pear
8×5 mm · ~0.9 ct

Emerald
7×5 mm · ~1.1 ct

Radiant
7×5 mm · ~1.1 ct

Princess
6×6 mm · ~1.3 ct

Asscher
6×6 mm · ~1.2 ct

Marquise
10×5 mm · ~1.1 ct

Heart
6×6 mm · ~1.1 ct

Trillion
7×7 mm · ~1.2 ct

Baguette
9×4.5 mm · ~1.1 ct

Lab-grown sapphire and ruby, drawn to scale with each other. Every shape is listed with its size in millimetres and an approximate weight.

Choosing your size

Go by millimetres. Two stones of the same carat weight can look quite different in size, depending on the gem and the shape. Elongated shapes such as oval, pear and marquise look larger face-up than a round stone of the same weight.
Exact weight. Each stone is cut to order, so its final weight can vary slightly from the estimate. If you add an IGI or GIA certificate, it records the measured weight.
Having it set. Our solitaire ring and pendant settings take stones up to 5 ct. For larger stones we make the setting to order and quote individually.
Matching a stone you already have? Send us its measurements in millimetres and we will cut to match. Ask us about a custom cut.