Diamond buying guide
Diamonds are graded using a set of internationally recognised standards. Understanding what those standards measure — and what they don't — makes it easier to know what you're looking at when you find a stone you like.
How diamonds are graded
The international standard for diamond grading is built around four characteristics: cut, colour, clarity, and carat weight. Known as the 4 Cs, the framework was developed by the Gemological Institute of America (GIA) in the mid-twentieth century and is now used by laboratories, jewellers, and buyers worldwide.
Each characteristic is assessed independently, and they interact with each other in ways that affect how a stone looks in a finished piece — set in a ring, pendant, or pair of earrings. The grades give you a vocabulary for understanding a stone; seeing it in person gives you the full picture.
Cut
Cut describes how a diamond's facets — the flat, polished faces — have been proportioned and aligned. It is a measure of craftsmanship, distinct from the shape of the stone, which is covered separately below.
The proportions determine how light behaves once it enters the stone. A diamond with well-executed proportions will reflect light back toward the viewer, producing brilliance (white light return), fire (dispersion of coloured light), and scintillation (sparkle as the stone or light source moves).
| Grade | What it means |
|---|---|
| Excellent | Proportions and finish that maximise light return. |
| Very Good | High light return with minor deviations from ideal proportions. |
| Good | Reflects a good proportion of light. Broader range of proportions. |
| Fair / Poor | Light escapes through the sides or bottom rather than the top. |
Formal cut grades apply most consistently to round brilliant diamonds. For fancy shapes, laboratories assess symmetry and polish but do not assign an overall cut grade in the same way.
Colour
Diamond colour is graded on a scale running from D at the colourless end to Z, where a yellow or brown tint is clearly visible. The scale was developed by the GIA and applies to white diamonds; fancy coloured diamonds — pinks, blues, yellows — are assessed under a separate system.
The difference between adjacent grades is subtle and becomes harder to detect once a diamond is set. How apparent the colour appears also depends on the setting metal, the lighting conditions, and the size of the stone — larger diamonds tend to show colour more visibly than smaller ones.
| Grade | Category | Description |
|---|---|---|
| D – F | Colourless | No detectable colour, even under magnification. |
| G – J | Near colourless | Colour difficult to detect unless compared directly with higher grades. |
| K – M | Faint | A slight warmth visible, particularly in larger stones. |
| N – Z | Very light – Light | Colour increasingly noticeable to the naked eye. |
Clarity
Clarity describes how free a diamond is from inclusions — internal characteristics such as tiny crystals, feathers, or clouds — and blemishes on its surface. These form during the diamond's growth underground and are assessed under 10× magnification by a trained grader.
An important distinction is whether inclusions are visible to the naked eye. Many diamonds with mid-range clarity grades are described as eye-clean, meaning their inclusions are not detectable without magnification. The visibility of inclusions also depends on their position within the stone, their size, and the diamond's shape.
Some diamonds undergo treatments — such as laser drilling to remove dark inclusions, or fracture filling to reduce the visibility of surface-reaching cracks. Treated stones should be disclosed as such, and any grading report will note whether treatments have been identified.
| Grade | Description |
|---|---|
| FL / IF | Flawless / Internally flawless. No inclusions visible under 10× magnification. |
| VVS1 / VVS2 | Very very slightly included. Inclusions extremely difficult to detect under magnification. |
| VS1 / VS2 | Very slightly included. Minor inclusions visible under magnification with some effort. |
| SI1 / SI2 | Slightly included. Noticeable under 10× magnification; often invisible to the naked eye. |
| I1 – I3 | Included. Inclusions visible to the naked eye and may affect transparency or durability. |
Carat
Carat is a unit of weight, not size. One carat equals 0.2 grams. The word derives from the carob seed, historically used as a counterweight for its consistency. Diamonds are weighed to two decimal places.
Because larger diamonds are found less frequently in nature, price per carat increases as stones get bigger. A 2ct diamond of equivalent quality to a 1ct stone will cost considerably more than twice the price.
Carat weight and physical size are related but not identical — the diameter a diamond presents to the eye depends on how the stone has been cut. Two diamonds of the same carat weight in different shapes will appear different in size.
Shape
Shape refers to the outline of the diamond — round, oval, pear, and so on. Unlike the 4 Cs, shape is not a quality grade; there is no better or worse. It is a matter of personal preference and how the stone will sit in the intended piece of jewellery.
Round brilliant is by far the most widely sold shape. Other shapes — collectively known as fancy cuts — have grown significantly in popularity in recent years.
| Shape | Description |
|---|---|
| Round brilliant | 58 facets designed to maximise brilliance. Accounts for the majority of diamonds sold. |
| Oval | An elongated form of the round brilliant. Can appear larger than its carat weight suggests. |
| Cushion | Square or rectangular with rounded corners and larger facets. Associated with antique and vintage styles. |
| Princess | Square or rectangular with numerous facets. A modern cut with strong brilliance. |
| Emerald | Rectangular with cut corners and step-cut facets. Emphasises clarity over brilliance. |
| Pear | Teardrop-shaped. Used in rings, pendants, and drop earrings. |
| Marquise | Elongated with pointed ends. Can make a stone appear larger than its weight. |
| Heart | A modified brilliant cut. Symmetry between the two lobes is central to the overall appearance. |
| Radiant | Square or rectangular with trimmed corners and a brilliant-style facet pattern. |
| Trilliant | Triangular, with pointed or rounded corners. Often used as side stones. |
Grading reports
A diamond grading report — sometimes called a certificate — is a document issued by an independent gemological laboratory confirming the characteristics of a specific stone. It records the 4 Cs alongside additional details such as symmetry, polish, and fluorescence (the extent to which a diamond emits a glow under ultraviolet light), and typically includes a diagram showing the position of any inclusions.
A grading report is issued by the laboratory, not the retailer, and is not a valuation — it does not state the monetary value of the diamond. It is a record of what the stone is, not what it is worth.
The main grading laboratories whose reports are widely recognised include:
| Laboratory | Notes |
|---|---|
| GIA | Gemological Institute of America. Developed the 4 Cs grading system. Reports are widely referenced internationally. |
| Gem-A | The Gem Testing Laboratory of Great Britain, operated by the Gemmological Association of Great Britain. |
| HRD Antwerp | Hoge Raad voor Diamant. Belgian laboratory with strong recognition across Europe. |
| Anchor Cert | Independent diamond certification recognised within the UK trade. |
| IGI | International Gemological Institute. Commonly used for laboratory-grown diamonds. |
Not all diamonds come with a grading report — particularly smaller stones or those already set in jewellery. Where no report exists, the characteristics described by the seller are based on their own assessment.
