Iolite size chart: carat to mm for every shape

By HeyGema™ · Updated November 5, 2026

In short

A 1 carat round iolite measures about 7.1 mm across, against 6.5 mm for a 1 carat diamond. Iolite is lighter than diamond (2.61 against 3.52), so the same weight gives a bigger stone, 10% wider across.

7.1 mm1 carat round iolite
2.61density, g/cm3
7 to 7.5hardness, Mohs

Carat to mm calculator

ShapeGemCarat

About 7.1 x 7.1 mm

An estimate for a well-cut stone. A deep or shallow cut faces up smaller or bigger.

Iolite size chart: carat to mm

Face-up length and width in mm of a well-cut iolite at each weight and shape. Iolite is lighter than diamond (2.61 against 3.52), so the same weight gives a bigger stone, 10% wider across.

Weight	Round	Oval	Cushion	Emerald cut	Pear	Princess
0.25 ct	4.5	5.6 x 3.8	3.9 x 3.9	4.9 x 3.5	5.9 x 3.8	3.8 x 3.8
0.5 ct	5.7	7.0 x 4.8	4.9 x 4.9	6.1 x 4.4	7.5 x 4.8	4.8 x 4.8
0.75 ct	6.5	8.0 x 5.5	5.6 x 5.6	7.0 x 5.0	8.5 x 5.5	5.5 x 5.5
1 ct	7.1	8.8 x 6.1	6.2 x 6.2	7.7 x 5.5	9.4 x 6.1	6.1 x 6.1
1.5 ct	8.2	10.1 x 7.0	7.1 x 7.1	8.9 x 6.3	10.8 x 7.0	7.0 x 7.0
2 ct	9.0	11.1 x 7.7	7.8 x 7.8	9.7 x 7.0	11.8 x 7.7	7.7 x 7.7
3 ct	10.3	12.7 x 8.8	8.9 x 8.9	11.2 x 8.0	13.5 x 8.8	8.8 x 8.8
4 ct	11.3	14.0 x 9.6	9.8 x 9.8	12.3 x 8.8	14.9 x 9.6	9.6 x 9.6
5 ct	12.2	15.1 x 10.4	10.6 x 10.6	13.2 x 9.4	16.1 x 10.4	10.4 x 10.4
10 ct	15.4	19.0 x 13.1	13.3 x 13.3	16.7 x 11.9	20.2 x 13.1	13.1 x 13.1

Length x width in mm. A round stone shows one number, its diameter.

mm to carat for a round iolite

If you know the size, this is the weight. Measure across the widest point of the stone, face up.

4 mm: Weight: 0.18 ct

5 mm: Weight: 0.35 ct

6 mm: Weight: 0.6 ct

7 mm: Weight: 0.95 ct

8 mm: Weight: 1.41 ct

9 mm: Weight: 2.01 ct

10 mm: Weight: 2.76 ct

12 mm: Weight: 4.77 ct

Diameter	Weight
4 mm	0.18 ct
5 mm	0.35 ct
6 mm	0.6 ct
7 mm	0.95 ct
8 mm	1.41 ct
9 mm	2.01 ct
10 mm	2.76 ct
12 mm	4.77 ct

Iolite against diamond

How wide a round iolite is next to a round diamond of the same weight.

0.5 ct: Iolite: 5.7 mm; Diamond: 5.1 mm; Difference: +0.5 mm

1 ct: Iolite: 7.1 mm; Diamond: 6.5 mm; Difference: +0.7 mm

2 ct: Iolite: 9.0 mm; Diamond: 8.1 mm; Difference: +0.9 mm

3 ct: Iolite: 10.3 mm; Diamond: 9.3 mm; Difference: +1.0 mm

5 ct: Iolite: 12.2 mm; Diamond: 11.0 mm; Difference: +1.2 mm

Weight	Iolite	Diamond	Difference
0.5 ct	5.7 mm	5.1 mm	+0.5 mm
1 ct	7.1 mm	6.5 mm	+0.7 mm
2 ct	9.0 mm	8.1 mm	+0.9 mm
3 ct	10.3 mm	9.3 mm	+1.0 mm
5 ct	12.2 mm	11.0 mm	+1.2 mm

Iolite properties

Density used in the charts: Value: 2.61 g/cm3 (range 2.61)

Hardness (Mohs): Value: 7 to 7.5

Refractive index: Value: 1.542 to 1.551

Property	Value
Density used in the charts	2.61 g/cm3 (range 2.61)
Hardness (Mohs)	7 to 7.5
Refractive index	1.542 to 1.551

Violet-blue. Light, so it faces up bigger than a diamond of the same weight.

Iolite at each weight

Questions

A 1 carat round iolite is about 7.1 mm across. An oval is 8.8 x 6.1 mm and a cushion cut 6.2 x 6.2 mm.

Yes. A 1 carat iolite is 7.1 mm and a 1 carat diamond 6.5 mm. Iolite is lighter than diamond (2.61 against 3.52), so the same weight gives a bigger stone, 10% wider across.

A 6 mm round iolite weighs about 0.6 carat. An 8 mm stone weighs about 1.41 carat.

A 5 carat round iolite is about 12.2 mm across (0.48 inches).

A carat is a weight, 0.2 grams. Gems differ in density, so a gram of a dense gem takes up less space than a gram of a light one. Dense stones face up smaller for their weight.