Alexandrite size chart: carat to mm for every shape

By HeyGema™ · Updated November 5, 2026

In short

A 1 carat round alexandrite measures about 6.3 mm across, against 6.5 mm for a 1 carat diamond. Alexandrite is denser than diamond (3.73 against 3.52), so it packs more weight into less space and faces up 2% narrower for the same weight.

6.3 mm1 carat round alexandrite
3.73density, g/cm3
8.5hardness, Mohs

Carat to mm calculator

ShapeGemCarat

About 6.3 x 6.3 mm

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

Alexandrite size chart: carat to mm

Face-up length and width in mm of a well-cut alexandrite at each weight and shape. Alexandrite is denser than diamond (3.73 against 3.52), so it packs more weight into less space and faces up 2% narrower for the same weight.

Weight	Round	Oval	Cushion	Emerald cut	Pear	Princess
0.25 ct	4.0	4.9 x 3.4	3.5 x 3.5	4.3 x 3.1	5.3 x 3.4	3.4 x 3.4
0.5 ct	5.0	6.2 x 4.3	4.4 x 4.4	5.4 x 3.9	6.6 x 4.3	4.3 x 4.3
0.75 ct	5.7	7.1 x 4.9	5.0 x 5.0	6.2 x 4.5	7.6 x 4.9	4.9 x 4.9
1 ct	6.3	7.8 x 5.4	5.5 x 5.5	6.9 x 4.9	8.3 x 5.4	5.4 x 5.4
1.5 ct	7.2	9.0 x 6.2	6.3 x 6.3	7.9 x 5.6	9.5 x 6.2	6.2 x 6.2
2 ct	8.0	9.9 x 6.8	6.9 x 6.9	8.7 x 6.2	10.5 x 6.8	6.8 x 6.8
3 ct	9.1	11.3 x 7.8	7.9 x 7.9	9.9 x 7.1	12.0 x 7.8	7.8 x 7.8
4 ct	10.0	12.5 x 8.6	8.7 x 8.7	10.9 x 7.8	13.2 x 8.6	8.6 x 8.6
5 ct	10.8	13.4 x 9.2	9.4 x 9.4	11.7 x 8.4	14.3 x 9.2	9.2 x 9.2
10 ct	13.6	16.9 x 11.6	11.8 x 11.8	14.8 x 10.6	18.0 x 11.6	11.6 x 11.6

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

mm to carat for a round alexandrite

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

4 mm: Weight: 0.25 ct

5 mm: Weight: 0.49 ct

6 mm: Weight: 0.85 ct

7 mm: Weight: 1.35 ct

8 mm: Weight: 2.02 ct

9 mm: Weight: 2.88 ct

10 mm: Weight: 3.95 ct

12 mm: Weight: 6.82 ct

Diameter	Weight
4 mm	0.25 ct
5 mm	0.49 ct
6 mm	0.85 ct
7 mm	1.35 ct
8 mm	2.02 ct
9 mm	2.88 ct
10 mm	3.95 ct
12 mm	6.82 ct

Alexandrite against diamond

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

0.5 ct: Alexandrite: 5.0 mm; Diamond: 5.1 mm; Difference: -0.1 mm

1 ct: Alexandrite: 6.3 mm; Diamond: 6.5 mm; Difference: -0.1 mm

2 ct: Alexandrite: 8.0 mm; Diamond: 8.1 mm; Difference: -0.2 mm

3 ct: Alexandrite: 9.1 mm; Diamond: 9.3 mm; Difference: -0.2 mm

5 ct: Alexandrite: 10.8 mm; Diamond: 11.0 mm; Difference: -0.2 mm

Weight	Alexandrite	Diamond	Difference
0.5 ct	5.0 mm	5.1 mm	-0.1 mm
1 ct	6.3 mm	6.5 mm	-0.1 mm
2 ct	8.0 mm	8.1 mm	-0.2 mm
3 ct	9.1 mm	9.3 mm	-0.2 mm
5 ct	10.8 mm	11.0 mm	-0.2 mm

Alexandrite properties

Density used in the charts: Value: 3.73 g/cm3 (range 3.73)

Hardness (Mohs): Value: 8.5

Refractive index: Value: 1.746 to 1.755

Property	Value
Density used in the charts	3.73 g/cm3 (range 3.73)
Hardness (Mohs)	8.5
Refractive index	1.746 to 1.755

A chrysoberyl that changes color between daylight and incandescent light. Stones over a carat are scarce.

Alexandrite at each weight

Questions

A 1 carat round alexandrite is about 6.3 mm across. An oval is 7.8 x 5.4 mm and a cushion cut 5.5 x 5.5 mm.

No. A 1 carat alexandrite is 6.3 mm and a 1 carat diamond 6.5 mm. Alexandrite is denser than diamond (3.73 against 3.52), so it packs more weight into less space and faces up 2% narrower for the same weight.

A 6 mm round alexandrite weighs about 0.85 carat. An 8 mm stone weighs about 2.02 carat.

A 5 carat round alexandrite is about 10.8 mm across (0.43 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.