Diamonds are unique in their mirror-like reflections - the technical term is adamantine lustre (and yes, hence the name diamond!). No other gem reflects back as much light as a diamond’s flat surface. And lots of flat surfaces make for lots of little mirrors - enter the faceted diamond. Diamond cutters use this characteristic to create the brightest most lively diamonds.

1.55 carat round brilliant diamond engagement ring by ronan campbell at DesignYard contemporary jewellery

The best diamonds are cut to maximise light return to the viewer, which we see as scintillation and brilliance. To make it work they have to align the facets to very specific angles, or they will literally miss the mark. Poorly cut diamonds will appear lifeless lacking brilliance, fire and scintillation. They may have dark centres, large culets, windows, or poorly polished surfaces, all of which interfere with light travelling through the gem and detract from the overall beauty of the diamond.

So now that we know why diamond cut is so important, let’s look at how it’s done.

Anatomy of a Diamond

First a little anatomy lesson. We’ll use a brilliant round diamond to help explain the different elements.

A diamond can generally be divided into three sections:

    • The shallow top section, called the crown

    • A narrow band at the circumference, called the girdle.

    • The steeper bottom section, called the pavilion.

Anatomy of a diamond - crown, girdle, pavillion

The crown is cut and polished into flat surfaces called facets. The main facet is the octagonal flat at the top of the gem, called the table. It is surrounded by star facets, bezel facets, and girdle facets. Each facet type should have the same shape and sit at specific angles to its neighbouring facet. All facets should meet at crisp lines and points.

The girdle may be slightly rough (called a bruted girdle), polished smooth, or may be polished into facets. Tip: if your diamond comes with a certificate this is where you will find the laser inscription with the certificate number.

laser inscription on girdle

The pavilion is cut and polished into two types of facets: the lower girdle facets and pavilion main facets. The very bottom of the diamond is called the culet. In a well cut diamond the facets align perfectly into a pointy culet. Less that ideal cut diamonds (and often also in antique gems) the culet will be a small flat instead, this is clearly visible through the table and considered less desirable in a modern diamond.

The crown facets are angled to draw surrounding light into the diamond and down to the pavilion facets. In the process they split the light into its different component colours (refraction) causing a colourful sparkle we call fire. Older diamond cuts often have steeper crowns, which cause more refraction and so more fire. Contemporary round brilliants draw more light into the pavilion for better brilliance. It is all a very fine balancing act!

Angles and light return for excellent cut

Very crisply polished surfaces are important to draw maximum light in. Poorly polished surfaces may show little surface pits or nicks at the facet or girdle edges. You may also find a streaky or chattered finish on a poorly polished facet. These will interfere with light travelling into the gem making for a darker gem.

The pavilion facets are angled to create both light and dark facets in a very carefully considered pattern - this is called scintillation. The idea is to maximise the light return, but also add depth and life by alternating light and dark facets. Just think if all facets returned light equally, it would be like turning on a headlamp, a fairly underwhelming experience.

Left to right poor, good and excellent cut diamonds

Round brilliant diamonds show how the quality of the diamond’s cut affects visual characteristics. The diamonds are positioned in left to right order: Poor cut, Good cut, Excellent Cut.

A poorly angled pavilion may cause a dark centre or worse: a window. An overly steep pavilion will shoot the light back past the eye resulting in a grey or dark centre. It also increases the carat weight of the gem, without any of the benefits of a larger facing gem. With an overly shallow pavilion the light spills out through the pavillion facets, creating a window that is effectively no different than a piece of glass. The gem may appear bigger as the spread of the round is larger, but you lose all the scintillation and brilliance, which is kinda the point of a diamond!

Other factors that affect brilliance and scintillation are discussed separately: colour and clarity.

Diamond certificates

So how do you know you have a well-cut diamond? Some flaws are easy to spot once you know what you are looking for. Others are only distinguishable with magnification or even laboratory equipment. Your safest bets are the diamond certificate and a trained eye.

A diamond with a certificate has been evaluated to exacting standards with laboratory equipment. The diamond report will grade a diamond’s surfaces for quality of polish, cut (it’s facets depth and angles), and symmetry (missing or extra facets, as well as a culet or table that is off-center will throw off all the diamonds facets).

GIA Certificate with cut specifications

Each is rated on a scale of Poor, Fair, Good, Very Good, or Excellent. The most sought after diamonds are Triple X diamonds - that is, a diamond that receives an Excellent rating on all three criteria.

Previously diamonds were cut by hand and a gem with a Triple X rating was a true test of craftsmanship, and also harder to find. With the advent of machine polishing triple X diamonds are much more prevalent.

niessing spannring round 1ct diamond engagement ring designyard contemporary jewellery gallery dublin ireland

Still a triple X gem is a reflection of the craftsmanship that went into making it. Top notch craftsmanship ensures that a diamond gives off maximum light and fire while having an attractive pattern.

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April 30, 2022 — Nicole van der Wolf
Tags: tips