How to Measure a Gemstone

What you need

• A digital caliper that reads to 0.01 mm. Plastic or stainless steel both work. Plastic jaws are kinder to soft gems and metal settings.
• Or a jeweller's spring gauge. These have fine pointed jaws that reach between prongs, which makes them handy for set stones.
• Good light and a loupe (10x magnification) to check where the jaws are touching.
• A soft cloth to clean the stone. Grease and dust add a surprising amount when you measure to a hundredth of a millimetre.

Before you start

1. Close the caliper jaws and press zero. Check it reads 0.00 mm again after opening and closing a few times.
2. Clean the stone and the jaws.
3. Hold the stone gently. Squeezing hard can tilt it and gives a slightly smaller reading on soft calipers.
4. Take every measurement at least twice. If two readings differ by more than 0.02 mm, take a third.

Length and width, by shape

Face-up measurements are always taken at the girdle, the widest edge of the stone.

Round

Measure across the girdle in one direction, then turn the stone 90 degrees and measure again. Well-cut rounds differ by less than 1%. Enter both in the calculator and it will average them.

Oval, cushion and emerald cut

Length is the longest dimension, end to end. Width is measured at right angles to the length at the widest point, usually the middle.

Pear and marquise

Length runs from the point to the far end (or point to point for a marquise). Width is the widest point across, which on a pear is near the rounded end, not the middle.

Heart

Length runs from the cleft at the top to the point at the bottom. Width is across the two lobes at their widest.

Triangle

Measure from one point to the middle of the opposite side for length, and the widest point across for width. For equilateral triangles both are close.

Princess, Asscher and square cushions

Measure side to side, not corner to corner. Corner to corner is about 40% larger for a square and will badly overstate the weight.

Cabochons

Measure the base outline as length and width. Depth is the height of the dome from the flat base to its highest point.

Depth

Depth is the total height of the stone, from the centre of the table on top to the culet (the point) at the bottom, or the keel (the ridge) on elongated shapes. Place one jaw flat on the table and the other on the culet, keeping the caliper square to the table. A tilted caliper reads too high.

If the culet is very sharp, close the jaws gently. Diamonds can mark soft metal jaws, and soft gems can chip if you clamp on a point.

Measuring a set stone

A set stone is harder because prongs, bezels and the mounting get in the way.

• Length and width: find a gap between prongs and measure girdle to girdle. A spring gauge with thin jaws is easiest. For a bezel, measure the visible stone and note that the true girdle is hidden; add the estimated overlap if you can see it.
• Depth: often impossible to measure directly. Some gauges can reach the culet through an open back. Otherwise, use the typical depth for the shape and accept a wider range. Our guide on estimating gem weight in a setting covers this in detail.

How much precision matters

Weight grows with every dimension at once, so small errors add up. Take an 8 x 6 x 4 mm oval sapphire, which our engine estimates at 1.54 ct. If each measurement is 0.1 mm too large (8.1 x 6.1 x 4.1 mm), the estimate becomes 1.62 ct, about 5% higher. For small stones the effect is bigger: a 3.0 mm round diamond, 1.8 mm deep, estimates at 0.099 ct, and at 3.1 x 1.9 mm it is 0.11 ct, about 11% more.

That is why a 0.01 mm caliper is worth having, and why our calculators always show a range rather than one precise number.

Common mistakes

• Measuring corner to corner on square and rectangular stones.
• Including the prongs in a set stone's width.
• Measuring depth at an angle, usually because the stone is tipped in a setting.
• Forgetting to zero the caliper, which shifts every reading by the same amount.
• Rounding too early. Record 6.47 mm, not "about 6.5". Round only at the end.

Recording your measurements

Write measurements the way labs do: length x width x depth in millimetres, for example 8.02 x 6.05 x 3.97 mm. For rounds, give the minimum and maximum diameter: 6.48 to 6.52 x 4.01 mm. Then enter them in the mm to carat calculator. For a quick size check without a stone in hand, the carat to mm finder works the other way.