How to Read a Diamond Certificate: GIA vs IGI vs HRD Explained

A diamond certificate (more properly called a "grading report") is not a guarantee of a beautiful diamond. It is an independent, scientific description of the stone's physical properties, produced by a qualified gemologist under controlled laboratory conditions. Understanding what each section means — and, critically, what it doesn't tell you — is essential for anyone buying a diamond of meaningful value.

The three main grading laboratories: GIA vs IGI vs HRD

Feature	GIA	IGI	HRD Antwerp
Founded	1931	1975	1973
Headquarters	Carlsbad, USA	Antwerp / Mumbai / NYC	Antwerp
Natural diamond grading	Strictest; gold standard	Historically 1–2 sub-grades looser than GIA	Broadly comparable to GIA
Lab-grown grading	Grading available; IGI dominates this market	Dominant standard for lab-grown	Less common for lab-grown
Report verification	report.gia.edu	report.igi.org	my.hrdantwerp.com
UK premium	Highest (10–20% vs IGI on same stated grade)	Lower	Mid
Our recommendation	Natural diamonds above 0.50ct	Lab-grown diamonds	European-cut or vintage natural stones

GIA — Gemological Institute of America GIA invented the modern diamond grading system in the 1950s and remains the strictest and most widely trusted laboratory for natural diamonds. A GIA certificate commands a price premium over IGI or HRD for the same stated grade, because the market has priced in the tighter grading. For a natural diamond above 0.50ct, we recommend GIA.

IGI — International Gemological Institute IGI is the largest laboratory by volume globally and is the dominant lab for lab-grown diamonds. Its grading for lab-grown stones is reliable and widely accepted. For natural diamonds, IGI has historically been observed to grade 1–2 sub-grades more generously than GIA — a stone graded VS1 G by IGI might grade VS2 H by GIA. This doesn't mean IGI is dishonest; it reflects different internal grade boundaries. It does mean you shouldn't compare an IGI-certified stone to a GIA-certified stone at the same stated grade without adjusting.

HRD Antwerp HRD (Hoge Raad voor Diamant — Diamond High Council) is a Belgian laboratory closely associated with the Antwerp diamond trade. It is a respected lab for European diamonds, particularly for older stones cut to Belgian proportions. Grading strictness is broadly comparable to GIA. Less common in the UK retail market but fully legitimate.

Anatomy of a GIA diamond grading report

The GIA report has changed in layout over the years but always contains the following sections:

Report number and laser inscription

Every GIA report has a unique number. This same number is laser-inscribed on the stone's girdle — barely visible to the naked eye, clear under a 10× loupe. Matching the number on the stone to the number on the certificate is the most reliable way to confirm that the right certificate accompanies the right stone.

Shape and cutting style

Describes the stone's outline (round, oval, cushion, pear, etc.) and how it was cut (brilliant, step-cut, mixed-cut). A "Round Brilliant" is the most commonly certified cut style.

Measurements

Three measurements in millimetres: minimum diameter × maximum diameter × depth (for round stones), or length × width × depth (for fancy cuts). These tell you the stone's physical size — which is distinct from its carat weight. Two 1.00ct round brilliants can have different face-up sizes if they are cut to different proportions.

Carat weight

Weight in carats to two decimal places. 1 carat = 0.200 grams. Weight is not the same as size (see Measurements above). A well-cut 0.95ct stone can have a larger face-up diameter than a poorly-cut 1.00ct stone.

Colour grade

Graded D (colourless) to Z (light yellow). D–F is colourless; G–J is near-colourless; K–M is faint colour; N–Z is noticeable colour. In practice, G and H are the most popular grades for engagement rings — the slight warmth is invisible in most settings and the price saving versus D–F is significant. Colour differences in round brilliants are hardest to detect below 1.00ct.

Clarity grade

Graded FL (flawless) — IF — VVS1 — VVS2 — VS1 — VS2 — SI1 — SI2 — I1 — I2 — I3. For most engagement rings, SI1 to VS2 is the practical range. VVS and FL/IF grades are paying for clarity that is invisible to the eye without laboratory equipment. SI2 and below can have inclusions visible to the naked eye — we check every SI stone individually before recommending it.

Cut grade (round brilliants only)

GIA grades cut on a five-point scale: Excellent, Very Good, Good, Fair, Poor. This combines Polish, Symmetry, and Proportions into a single headline grade. We recommend only Excellent or Very Good cut grades. An Excellent or Very Good cut visually outperforms a poorer-cut stone of higher colour and clarity.

Fluorescence

Describes whether and how strongly the stone glows under UV. See our FAQ below for detail.

Clarity plot

A diagram of the stone with inclusions marked. Use this to understand where inclusions sit — a central-table inclusion is more visible than one under a prong.

Proportion data (key figures)

• Table % — the diameter of the table facet as a percentage of the girdle diameter. For round brilliants, 54–60% is ideal.
• Depth % — total depth as a percentage of average girdle diameter. 59–63% is ideal.
• Crown angle and pavilion angle — the slopes of the crown and pavilion. Specific combinations produce maximum fire and brightness.

What the certificate doesn't tell you

The certificate describes the stone under controlled laboratory lighting. It doesn't tell you how the stone will look in your setting, in natural light, or against your partner's skin tone. It doesn't tell you whether an SI1 inclusion happens to be in a position that a prong will cover, or centred on the table where it will be immediately visible.

This is why we look at every stone in person. Two GIA-certified stones with identical grades can look noticeably different from each other. Our job is to find the one that looks better than its certificate suggests, within your budget.

Come in to 7 Murray Place, Stirling, and we'll show you diamonds alongside their certificates, explain exactly what you're looking at, and let you compare in daylight conditions. Call 01786 462799 to book a diamond viewing appointment.