Diamond Grading vs Gemstone Grading

Diamond Grading: The Industry Standard

Having a diamond graded is extremely common practice and, for most diamonds of any significant size or value, it is something buyers should expect as standard. The great majority of diamonds we sell at Cheshires come with a certificate from one of the leading international laboratories. The two you will encounter most often are GIA (the Gemological Institute of America) and IGI (the International Gemological Institute), though we also stock stones graded by other respected laboratories.

A diamond certificate gives you a precise record of the stone's characteristics. The weight in carats and the measurements in millimetres are objective, factual figures that any competent instrument can verify. Everything else on the certificate, however, is a matter of expert judgement.

Colour, clarity, cut grade, polish and symmetry are all assessed by trained gemologists, and while the leading laboratories operate to rigorous standards with multiple graders checking each stone, the results are professional opinions rather than absolute scientific facts. Two respected laboratories can, and sometimes do, arrive at slightly different conclusions for the same diamond. This is not a failing of the system; it is simply the nature of grading something as nuanced as a diamond.

What a certificate does do, and this is its great value, is give you a reliable, independent reference point. It means the stone you are buying has been assessed by professionals with no commercial interest in the outcome, and that assessment is documented and permanent.

The Leading Diamond Laboratories

While we stock diamonds with certificates from a range of laboratories, the two names you will see most often in our collection are GIA and IGI. Both are internationally recognised and widely respected within the trade.

Gemstone Grading: A Different World

Coloured gemstones operate under an entirely different set of conventions. Unlike diamonds, it is not common practice for coloured gemstones to be graded, and this is not an oversight. The cost of obtaining a certificate from one of the world's top laboratories is significant, and for many gemstones it simply would not be proportionate. As a general rule, you would only seek a certificate for a stone of considerable value, one where the investment in independent verification is clearly justified.

The purpose of a gemstone certificate is also quite different to that of a diamond certificate. Where a diamond report primarily grades the stone's quality characteristics, a gemstone certificate is principally sought to establish two things: where the stone is from, and whether it has been treated. Both have a profound impact on value.

Origin and Treatment: Why They Matter So Much

The origin of a gemstone, meaning the mine or region it came from, carries significant weight in the trade. A Burmese ruby, a Ceylon or Kashmir sapphire, or a Colombian emerald will command a very different price to a stone of similar appearance from another source, because certain origins have historically produced material of exceptional colour and quality, and the prestige of that provenance is built into the value.

Treatment is equally important. The vast majority of coloured gemstones on the market have been treated in some way to enhance their appearance, and this is widely accepted practice. The most common treatments are heat treatment, which improves colour and clarity in sapphires and rubies, and oiling, which is used extensively with emeralds to reduce the visibility of internal fractures. What matters to both buyer and seller is that any treatment is disclosed clearly and honestly, because an untreated stone of fine quality is considerably rarer and more valuable than a treated one of the same appearance.

The Leading Gemstone Laboratories

The most respected gemstone laboratories in the world tend to be Swiss, reflecting both the country's long tradition in watchmaking and gemology, and the rigorous scientific standards that tradition demands. When it comes to establishing origin and treatment for high-value coloured stones, the names below represent the gold standard of the industry.

Beyond the Swiss trio, we also stock gemstones accompanied by certificates from other well-regarded laboratories including GCS (Gem and Crystal Sciences), Gem and Pearl Lab, AIG (Antwerp International Gemological Laboratories) and Bellerophon, among others. The appropriate laboratory for any given stone will depend on its type, value and the specific information being sought.

For a truly exceptional stone of great rarity and value, it is not unusual to see certificates from more than one top laboratory. This is not duplication for its own sake: it reflects the fact that for the most significant stones, corroboration from multiple independent sources provides additional confidence for both buyer and seller, and is often expected in the market for that calibre of gemstone.

Diamond vs Gemstone Grading at a Glance

A Growing Appetite for Provenance

Gemstone certification is becoming increasingly popular, and this reflects a broader shift in how buyers relate to their jewellery. Customers today want to understand the full story of their stone: where it was found, how it was formed, what journey it has made from mine to setting. A certificate from a respected laboratory is the most reliable way to answer those questions.

This growing interest in provenance is particularly evident with rubies, sapphires and emeralds, where origin can make a very significant difference to both value and meaning. A Burmese ruby or a Ceylon or Kashmir sapphire certified by GRS or Gübelin carries with it not just a colour grade but a story, one that connects the wearer to one of the world's great gemstone-producing regions and to centuries of history.

At Cheshires, we welcome these conversations. Whether you are considering a diamond with a GIA certificate or a Kashmir sapphire with Swiss laboratory documentation, our team is here to help you understand exactly what you are looking at and what makes it special. Knowledge is part of what makes fine jewellery worth owning.