Gemstone Size Guide

Topsy Jewels — Complete Guide

Gemstone Size Guide —
See Your Stone in Millimetres

How big is a 6 mm stone? What does 1 carat look like? Use our charts to turn millimetres into carats for every shape, see stones at actual size, and choose the right size for rings, pendants and earrings.

mm to carat charts
9 stone shapes
Actual-size preview
Gem weight guide & FAQs
Section 01 — Reference

Round Stone mm to Carat Chart

Carat measures weight; millimetres measure the size you actually see. This chart shows the approximate weight of a well-cut round diamond at each diameter, the matching weight in moissanite, and what that size is usually used for.

Diameter Diamond weight Moissanite weight Typical use
3 mm 0.10 ct 0.09 ct Accent stones, dainty studs
3.8 mm 0.20 ct 0.18 ct Accent stones, dainty studs
4.1 mm 0.25 ct 0.23 ct Accent stones, dainty studs
4.5 mm 0.33 ct 0.30 ct Accent stones, dainty studs
5 mm 0.45 ct 0.41 ct Everyday studs, petite pendants
5.2 mm 0.50 ct 0.46 ct Everyday studs, petite pendants
5.5 mm 0.60 ct 0.55 ct Everyday studs, petite pendants
5.8 mm 0.70 ct 0.64 ct Everyday studs, petite pendants
6 mm 0.75 ct 0.68 ct Solitaire rings & pendants
6.5 mm 1.00 ct 0.91 ct Solitaire rings & pendants
6.8 mm 1.15 ct 1.05 ct Solitaire rings & pendants
7 mm 1.25 ct 1.14 ct Solitaire rings & pendants
7.4 mm 1.50 ct 1.36 ct Statement solitaires
7.8 mm 1.75 ct 1.59 ct Statement solitaires
8.2 mm 2.00 ct 1.82 ct Statement solitaires
8.8 mm 2.50 ct 2.27 ct Statement solitaires
9.4 mm 3.00 ct 2.73 ct Cocktail & centre-piece stones
10.2 mm 4.00 ct 3.64 ct Cocktail & centre-piece stones
11 mm 5.00 ct 4.55 ct Cocktail & centre-piece stones
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Coloured gemstones weigh differently. A 6.5 mm ruby weighs about 1.15 ct, but a 6.5 mm emerald only about 0.75 ct, even though both look the same size. See the gem weight guide below.
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Section 02 — Visual

Stone Sizes at Actual Size

These circles are drawn to scale in millimetres. At 100% browser zoom they are close to real life on most screens. Hold a ruler up to the 10 mm circle to check your screen.

3 mm0.10 ct
4 mm0.25 ct
5 mm0.45 ct
6 mm0.75 ct
6.5 mm1.00 ct
7 mm1.25 ct
8 mm2.00 ct
9 mm2.50 ct
10 mm4.00 ct
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Section 03 — Shapes

Fancy Shape Size Charts

Elongated shapes are measured length × width. Weights are approximate for diamond; multiply by the gem factor in Section 04 for coloured stones. Oval, pear and marquise look bigger than a round of the same weight.

Oval

Size Diamond weight
5 × 3 mm 0.25 ct
6 × 4 mm 0.50 ct
7 × 5 mm 0.75 ct
8 × 6 mm 1.25 ct
9 × 7 mm 1.75 ct
10 × 8 mm 2.50 ct

Pear

Size Diamond weight
5 × 3 mm 0.20 ct
6 × 4 mm 0.40 ct
7 × 5 mm 0.65 ct
8 × 6 mm 1.00 ct
9 × 6 mm 1.25 ct
10 × 7 mm 1.75 ct

Cushion

Size Diamond weight
4 mm 0.35 ct
5 mm 0.60 ct
6 mm 1.00 ct
7 mm 1.50 ct
8 mm 2.25 ct

Emerald Cut

Size Diamond weight
5 × 3 mm 0.25 ct
6 × 4 mm 0.50 ct
7 × 5 mm 1.00 ct
8 × 6 mm 1.50 ct
9 × 7 mm 2.25 ct

Princess

Size Diamond weight
3.5 mm 0.25 ct
4.5 mm 0.50 ct
5.5 mm 1.00 ct
6 mm 1.25 ct
6.5 mm 1.50 ct

Heart

Size Diamond weight
5 mm 0.50 ct
6 mm 0.75 ct
6.5 mm 1.00 ct
7 mm 1.25 ct
8 mm 1.75 ct

Trillion

Size Diamond weight
5 mm 0.35 ct
6 mm 0.50 ct
7 mm 0.90 ct
8 mm 1.25 ct

Marquise

Size Diamond weight
6 × 3 mm 0.25 ct
8 × 4 mm 0.50 ct
10 × 5 mm 1.00 ct
12 × 6 mm 1.50 ct
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Section 04 — Gemstones

Why Gems of the Same Size Weigh Differently

Every gemstone has its own density (specific gravity). Denser stones weigh more at the same size. Multiply the diamond weight from the charts above by the factor below to estimate the carat weight of each gem.

Gemstone Specific gravity Factor vs diamond At the same mm size
Ruby & Sapphire 4.00 1.14× Heavier - a 6.5 mm stone is about 1.15 ct
Garnet 3.95 1.12× Heavier than diamond
London & Swiss Blue Topaz 3.53 1.00× About the same as diamond
Peridot 3.34 0.95× Slightly lighter
Tanzanite 3.35 0.95× Slightly lighter
Moissanite 3.21 0.91× Slightly lighter - same look, less weight
Pink Tourmaline 3.06 0.87× Lighter
Aquamarine & Morganite 2.72 0.78× Lighter
Emerald 2.72 0.77× Lighter - a 6.5 mm stone is about 0.75 ct
Amethyst & Citrine 2.65 0.75× Lighter
Black Onyx 2.60 0.74× Lighter
Rainbow Moonstone 2.57 0.73× Lighter
Opal & Fire Opal 2.15 0.61× Much lighter - size by mm, not carat
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Example: a 7 × 5 mm oval is about 0.75 ct in diamond. In amethyst it weighs about 0.75 × 0.75 = 0.56 ct; in blue sapphire about 0.75 × 1.14 = 0.86 ct. Both look exactly the same size.
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Section 05 — Advice

Choosing the Right Size

The best stone size depends on the type of jewelry, your hand and how you like to wear it.

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Engagement & promise rings

6–8 mm centre stones are the most popular. Add a halo to make a 6 mm stone look closer to 8 mm.

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Stud earrings

4–5 mm for everyday and second piercings, 6–7 mm for a classic statement, 8 mm+ for evening.

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Pendants

6–8 mm reads well on a fine chain. Pear and oval pendants of 8 × 6 mm and up have real presence.

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Match your finger

Slim fingers (US 4–6) suit 5–7 mm stones; larger fingers (US 8+) carry 7–9 mm stones in proportion.

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Elongated shapes look bigger

Oval, pear and marquise cover more surface and lengthen the finger, giving more look for the weight.

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Buying a gift?

When in doubt, choose a medium size: 5–6 mm studs or a 6–7 mm pendant suit almost everyone.

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Section 06 — Questions

Frequently Asked Questions

For everyday stud earrings, 4 mm to 6 mm round stones are the sweet spot. They catch the light without looking oversized, and they sit comfortably under hats, scarves and phone calls. Go up to 6.5–7 mm for evening wear or if you prefer a bolder look.
A 1 carat round diamond measures about 6.5 mm across. Moissanite of the same size weighs about 0.9 ct. Because gemstones have different densities, a 6.5 mm sapphire or ruby weighs about 1.15 ct, while a 6.5 mm emerald weighs only about 0.75 ct. The face-up size is what you actually see, so we list our stones by millimetre size.
Carat is a measure of weight, not size. Two stones of the same weight can look very different depending on the gem type and cut. Millimetres tell you exactly how big the stone looks when it is worn, which is what matters when choosing jewelry.
Elongated shapes such as oval, pear and marquise spread their weight over a larger surface, so they look bigger than a round stone of the same carat weight. They also make fingers look longer and slimmer. Emerald cuts and cushions tend to look slightly smaller than rounds of the same weight because more of the weight sits in the depth of the stone.
Most engagement ring centre stones fall between 6 mm and 8 mm (about 0.75 to 2 ct in diamond weight). On slimmer fingers (US sizes 4–6) a 6–6.5 mm stone looks generous; on larger fingers a 7–8 mm stone keeps the same proportion. A halo setting adds about 2 mm to the visible size.
A halo of small accent stones adds roughly 1.5–2.5 mm around the centre stone, so a 6 mm centre with a halo can look like an 8 mm stone from a distance. It is one of the most affordable ways to get a bigger look.
It is close on most screens at 100% zoom, but screens differ. To check yours, hold a bank card (85.6 mm wide) against your screen and compare it to a ruler, or measure the 10 mm circle with a ruler. For a precise feel, cut paper circles using the millimetre sizes in the chart.
Many of our rings and pendants are available in several stone sizes, shown as a size option on the product page. If you need a size that isn't listed, contact us and we will let you know what is possible for that design.

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