Diamond Certification Explained: GIA, IGI, and What the Grades Mean

At some point in the diamond-buying process, most people run into a wall of unfamiliar terminology. The 4Cs make sense in theory, but then grading reports enter the picture, and suddenly there are two institutions, unfamiliar abbreviations, and fine print that isn't obvious to anyone outside the industry.

For diamonds that carry the most weight in a purchase, diamond certification is the document that makes all of that legible. A grading report is issued by an independent laboratory with no stake in the sale, which means the grades reflect what a stone actually is, not what a retailer wants it to be. For buyers ready to commit, it's the clearest verification available. Below, we go over the difference between certified and uncertified diamonds, what a diamond certification covers, and why it matters.

Certified vs. Uncertified Diamonds

What Diamond Certification Is and Why It Matters

Think of a grading report as a stone's record, issued not by the seller but by an independent gemological institution whose job is to evaluate diamonds (both lab-grown and natural) without bias. A grade assigned by GIA or IGI reflects the stone's actual characteristics, not the asking price or the retailer's interests.

For buyers shopping in person, a trusted jeweler can fill some of that role. For buyers shopping online, the grading report does the work instead. It's what makes it possible to compare a VS2, G color, Excellent cut from one retailer to the same grades from another and know you're working from the same baseline.

That said, it’s most common to find certifications for diamonds above a certain carat weight and size (typically those used as center stones, solitaire pendants, or other larger featured pieces). It is not industry practice to provide individual certifications for very small stones, including side or accent diamonds. This is because the cost of grading a very small stone often outweighs the practical benefit, and those stones are evaluated for quality at the sourcing level rather than individually documented. 

If a piece features a certified center stone alongside smaller uncertified accent diamonds, that's completely normal and exactly what you'd expect from any reputable fine jeweler.

What a Diamond Certificate Covers

Every grading report covers the same core categories, regardless of which institution issues it:

Cut

Of the four Cs, cut has the most visible impact on how a diamond looks, which is why it’s often seen as the most important factor. Cut describes how a stone has been shaped and faceted to interact with light, graded from Excellent to Poor. A well-cut diamond reflects light brilliantly regardless of its color or clarity grade. A poorly cut stone, on the other hand, can look flat and lifeless even with impressive numbers in every other category.

Color

Color measures how colorless a diamond is, on a scale from D (completely colorless) to Z (a noticeable yellow or brown tint). The differences between adjacent grades are nearly invisible in person, but each step up the scale comes with a noticeably higher price tag. Most buyers land somewhere in the G to J range, where a stone reads as colorless in most settings without paying the premium of a D or E.

Clarity

Clarity grades the presence of internal inclusions and surface blemishes, from Flawless at the top to I3 at the bottom. Because grading happens under 10x magnification, the differences between most grades above SI1 are invisible in everyday wear. Where clarity starts to matter visibly is in the I1 to I3 range, where inclusions become detectable to the naked eye.

Carat

Carat measures a diamond's weight, which corresponds with size. Because of this, it tends to be the first number people ask about. But carat weight alone doesn't tell you how large a diamond actually looks. A well-cut stone will appear bigger and brighter than a heavier diamond with a poor cut, because cut is what determines how that weight translates into size and sparkle in person.

Beyond the 4Cs

A grading report also includes the diamond's physical measurements in millimeters, a fluorescence rating indicating how the stone reacts under ultraviolet light, and a unique report number laser-inscribed on the diamond's girdle, allowing the stone to be verified directly against its documentation. For lab-grown diamonds, the report additionally notes the growth method, either CVD (Chemical Vapor Deposition) or HPHT (High Pressure High Temperature).

GIA and IGI At a Glance

The two institutions buyers encounter most often are the Gemological Institute of America (GIA), a non-profit founded in 1931 that developed the 4Cs framework and is widely regarded as the benchmark for natural diamonds. As of late 2025, the GIA no longer certifies lab-grown diamonds. The International Gemological Institute (IGI), a for-profit organization founded in 1975 has become the dominant certifying institution for lab-grown diamonds. Both institutions produce credible, independent grading reports, but they differ in their history, market focus, and grading approach. 

At Milano, both certifications are used and matched to the type of diamond to ensure the most appropriate standard is applied to each piece. For a detailed breakdown of how they compare and which carries more weight for your specific purchase, see our guide on IGI vs GIA.

Shop Certified Diamonds at Milano

Diamond certification doesn’t make the stone more beautiful. But it does confirm that what you're buying is exactly what it's represented to be, and is graded independently, documented accurately, and verified against a standard that no retailer controls. At Milano, every eligible diamond comes with a GIA or IGI grading report included, so you can review the documentation before you decide. Browse Milano's certified lab-grown diamond collection or explore our natural diamond jewelry to find the perfect piece for you.

Frequently Asked Questions

Why don't all diamonds in a piece of jewelry come with a certificate?

Certification is most valuable for larger, higher-value stones where a buyer needs independent verification of what they're purchasing. For very small accent or side diamonds, the cost of grading each stone individually doesn't reflect its size or price, and this isn't standard practice in the industry. Those stones are still selected for quality, just evaluated at the sourcing level rather than individually documented. A certificate for the center stone, where present, is what matters most.

Does diamond certification affect the price? 

Yes, somewhat. Certified diamonds typically cost slightly more than uncertified stones because the grading process itself incurs costs, and the documentation provides verified quality assurance that protects the buyer. A grading report also provides the paperwork needed for insurance purposes, which extends its value well beyond the day you buy.

Can a diamond certificate be faked or altered? 

It's very difficult to do convincingly. Both GIA and IGI offer online verification tools where buyers can enter a report number and confirm the details match the physical document. Because the same report number is also laser-inscribed on the stone's girdle, the diamond and its certificate can be cross-referenced directly, making substitution or alteration easy to detect.

What happens if I lose my diamond certificate? 

A replacement can be requested directly from GIA or IGI using the original report number, which is typically laser-inscribed on the diamond's girdle. As long as that inscription is intact, the stone can be re-matched to its documentation. Keeping a digital copy alongside the physical certificate is a simple safeguard to give you peace of mind.