Diamond Grading Explained:
Why Diamond Certificates Differ, How Diamonds Are Graded, and Which Laboratory Is Best
When buying a diamond, one of the most important documents you’ll encounter is the diamond grading certificate. This report is issued by an independent gemmological laboratory and describes a diamond’s quality — its cut, colour, clarity, carat weight, and other key characteristics.
However, not all grading laboratories are the same. If you’ve ever wondered why two diamonds with similar-looking certificates can vary so much in quality or price, the answer often lies in who graded the stone.
In this article, we’ll explain:
How diamonds are graded
Why grading laboratories differ
The key differences between GIA, IGI, and DCLA
Which grading laboratory is considered the most reliable
How Diamonds Are Graded
Diamond grading is performed by trained gemmologists using specialised equipment and controlled lighting conditions. Each stone is assessed independently and assigned grades based on the 4Cs:
Cut – how well the diamond has been proportioned and finished
Colour – how colourless (or tinted) the diamond appears
Clarity – the presence of internal inclusions or surface blemishes
Carat Weight – the diamond’s weight
Additional details may include polish, symmetry, fluorescence, proportions, and laser inscriptions.
While this sounds objective, diamond grading still involves human judgement, especially when assessing colour and clarity. This is why different laboratories — and even different graders — can sometimes produce different results for the same diamond.
Why Diamond Grading Laboratories Differ
Each laboratory has:
Its own grading standards and tolerances
Different training systems and peer review processes
Different commercial models (some work closely with manufacturers, others do not)
Some labs are known for very strict grading, while others are considered more lenient. Even a difference of one colour or clarity grade can significantly affect a diamond’s value.
This makes the choice of grading laboratory extremely important.
GIA – Gemological Institute of America
Reputation: Global gold standard
Founded: 1931
Headquarters: USA
The GIA is widely regarded as the most respected and consistent diamond grading laboratory in the world. It actually created the modern diamond grading system, including the D–Z colour scale and the clarity grading terminology used today.
Key Characteristics of GIA:
Extremely strict and conservative grading
Multiple graders assess each diamond independently
Strong internal checks and balances
Completely independent and non-commercial
Because of this consistency, GIA-graded diamonds are trusted worldwide and often command higher market values than diamonds graded by other labs with the same stated grades.
In the trade:
If a diamond is GIA-graded, jewellers and dealers can be confident that the grades are accurate — sometimes even slightly conservative.
IGI – International Gemological Institute
Reputation: Widely used, especially for lab-grown diamonds
Founded: 1975
Headquarters: Belgium
IGI is one of the largest gemmological laboratories in the world and is commonly used for lab-grown diamonds and commercial jewellery.
Key Characteristics of IGI:
Faster turnaround times
Widely accepted in retail jewellery
Often used by large manufacturers
More variable grading strictness
IGI grading can be slightly more lenient than GIA, particularly in colour and clarity. This doesn’t mean IGI diamonds are “bad” — but a diamond graded as, for example, G colour by IGI may sometimes receive an H colour by GIA.
In the trade:
IGI is very common for lab-grown diamonds and mass-market jewellery, but natural diamonds graded by IGI are often priced lower than equivalent GIA stones due to grading perception.
DCLA – Diamond Certification Laboratory of Australia
Reputation: Extremely strict, boutique-level grading
Founded: Australia
The DCLA is Australia’s most respected diamond grading laboratory and is known within the trade for its exceptionally strict standards — in some cases even stricter than GIA.
Key Characteristics of DCLA:
Very conservative grading
Detailed reports with advanced cut analysis
Strong emphasis on precision and transparency
Highly regarded by independent jewellers
DCLA reports are especially valued by clients who want maximum confidence in what they’re buying. It’s not uncommon for a DCLA-graded diamond to receive lower grades than the same stone graded elsewhere — but those grades are extremely reliable.
In the trade:
A DCLA certificate is a strong indicator of honesty and quality, particularly for high-end natural diamonds.
Which Diamond Grading Laboratory Is the Best?
There is no single “best” lab for every situation, but in terms of strictness and trust:
Most internationally recognised:
→ GIA
Most commonly used (especially for lab-grown):
→ IGI
Strictest and most conservative (Australia):
→ DCLA
At our studio, we always look beyond the certificate itself. We assess the actual diamond, its beauty, light performance, and suitability for the design — not just the letters on a report.
Why Diamond Grading Matters to You
Understanding diamond grading helps you:
Compare diamonds accurately
Avoid overpaying for inflated grades
Choose stones with genuine long-term value
Buy with confidence
A certificate is a tool — not a guarantee of beauty. Two diamonds with identical grades can still look very different in real life.
That’s why working with an experienced, independent jeweller matters.
Our Approach to Diamond Selection
We personally inspect and hand-select every diamond we supply — whether natural or lab-grown. We explain grading honestly, guide you through the differences between laboratories, and help you choose the best value and most beautiful stone for your budget and design.
If you ever have questions about a diamond certificate — or want a second opinion — we’re always happy to help.