What Diamond Grading Labs Actually Do

A diamond certificate looks official, because it is. But few buyers know what happens behind it. Understanding the process helps you use the report wisely.

Step 1: identification

The first job is to confirm that the stone is diamond, and whether it is natural or laboratory-grown, and whether it has been treated. Specialised instruments measure how the stone interacts with light and radiation. Natural diamonds, laboratory-grown diamonds and imitations all have different signatures.

Step 2: measuring

The stone is weighed on a precise balance to a hundredth of a carat or finer. Its dimensions are measured with precision tools, and its proportions are recorded.

Step 3: grading the four Cs

Trained graders examine the stone under magnification and controlled lighting.

• Clarity: examined under 10x magnification, noting the type, size and position of features.
• Colour: compared against master stones in a standard environment, with the stone face-down.
• Cut: assessed from the proportions and the finish, with instruments that measure angles and symmetry.

Many laboratories have several graders look at a stone independently, and the grade reflects their agreement.

Step 4: finish and fluorescence

Polish and symmetry are examined under magnification. Fluorescence is checked under a UV lamp and recorded.

Step 5: the report

The results are compiled into a report with a unique number, and in many cases the stone is laser-inscribed on its girdle with the number. The report may include a plot and a proportion diagram. See how to read a diamond certificate step by step.

What laboratories do not do

• They do not value the stone. A report is not an appraisal.
• They do not certify the jewellery. Only the diamond is graded, not the metal or the setting.
• They do not grade stones once mounted in the same way. Grading is done on loose stones for accuracy.
• They do not promise that two laboratories will give the same grade. Standards differ in how strictly they apply the scales, as discussed in diamond certificates: IGI, GIA, SGL and what they mean.

Why the grade is an opinion

Grading involves human judgement, though aided by instruments. That is why a stone may receive adjacent grades from different laboratories. The scales are fixed, but the borderlines are a matter of judgement.

Re-grading

Stones are sometimes submitted for re-grading, and the result can differ by a grade. Always rely on the report you hold for the stone you are buying, and verify it with the issuing laboratory.

How to use this

1. Insist on a report from a recognised laboratory for significant stones.
2. Check the number online.
3. Look for the inscription on the girdle.
4. Compare the stone in front of you with its description.

Small stones

Melee are not graded individually. See melee diamonds.

Why laboratory-grown stones have their own reports

Laboratories issue separate reports for laboratory-grown diamonds, and those reports state the origin plainly. This matters for transparency: the report you hold should say whether the stone is natural or grown. If a report does not state origin, ask the seller to explain.

Seeing the process at work

If you would like to see a stone, its inscription and its report together, book an appointment. To see how stones are described in finished jewellery, browse our diamond rings.