Diamond Fundamentals

Diamond Cut

Find out what makes a diamond sparkle through our rich and detailed information on diamond cut. Learn about cut grading and the anatomy of a diamond.

What is diamond cut?

As most gemmologists would agree, diamond cut is the most important characteristic to prioritise when choosing a diamond. The best natural diamond cut is graded Excellent by GIA, and the best lab diamond cut is graded Ideal by IGI. Excellent (GIA) and Ideal (IGI) cut diamonds are made up of the most precise angles, symmetry, proportion, balance of facets and best polish that a highly skilled diamond cutter can achieve. Precise diamond cutting will equate to a diamond that reflects light perfectly, so that the radiance, fire and sparkle of the stone is at its absolute best.

Hundreds of years of study and experimentation have allowed for an increased scientific knowledge of the way diamonds work, and how light reflects and refracts within the crystals. Now, we are able to maximise their natural brilliance through diamond cutting with extremely precise measurements.

The anatomy of a diamond and facet names

There are eight types of facets that make up a diamond cut, each with their own role to play and specific calculations. As you usually view a diamond set within a piece of jewellery from the top, the table may seem like the most important part of the diamond. It is, however, a perfect, harmonious balance between each part that equates to the best diamond cut.

  • table
  • upper girdle
  • star
  • bezel
  • girdle
  • pavilion
  • lower girdle
  • culet
Diagram showing the different facets of a diamond

Diamond cut and reflection

To fully understand the way diamond cuts work, it’s helpful to have an idea of the way light behaves inside a diamond. There are three main ways that a diamond’s sparkle can be categorised.

, otherwise known as fire, refers to the coloured light that has been split into its full rainbow spectrum. While white light enters and exits through the table, flashes of colour are visible when light is propelled out from the centre of the stone and through the crown.

is how bright a diamond is – that is, how much white light it exudes and reflects out of the stone. The wide table at the top of a diamond takes in natural light, which is then split apart into the full spectrum of colours. These rebound and reflect within the diamond before being projected upwards and out through the table, exiting once again as pure white light.

is how the diamond glistens and twinkles – it is the way in which light bounces and plays off the facets of a diamond when it is moved. When a diamond is well cut, light will emanate from every angle, and provide the mesmerising sparkle we all know and love.

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That’s why we have diamond and design experts on hand, who are here to guide you through every step - from understanding what carat, color, cut and clarity mean, to designing the perfect ring.

How is diamond cut graded?

Diamond cut is graded on a sliding scale from Excellent to Poor, when graded by GIA. The IGI grade diamonds from Ideal to Fair, essentially following the same grading criteria. GIA are regarded the best graders for natural diamonds, and IGI the best graders for lab diamonds.

It can be a little confusing to know the difference between the cut grades used by different grading laboratories - use the table below to compare GIA vs IGI cut grades.

The cut grade of a diamond is regarded as the most important of the 4Cs, and gemmologists agree you should aim for an Excellent cut, or at the least a Very Good cut, for a bright, fiery diamond. If looking at a lab diamond, the equivalent grade to aim for would be Ideal. A poorly cut diamond may be too deep or too shallow, both largely diminishing the diamond’s potential to sparkle.

Getting each facet and angle perfectly proportioned is critical to creating the best diamond cut, Excellent. Facets that do not line up at the correct angles can cause light leakage, whereby light is reflected out of the pavilion or culet rather than the table and crown (the top section of the diamond). This equates to a dull stone with a dark centre.

The best cut diamonds, graded Excellent to Very Good (GIA) or Ideal (IGI), will reflect light out through the table and crown, with eye-catching flashes of white and coloured light.

GIA Cut Grade (Natural) IGI Cut Grade (Lab Grown) Grade Description
Excellent Ideal The best diamond cut grade, an Excellent cut will offer optimum fire and brilliance, with crisp facets and a smooth polish. This will appear bright, icy and extremely sparkly
Very Good Excellent You may not notice a huge difference between Very Good and Excellent, but the diamond will reflect slightly less light through the table. We recommend this is the lowest cut grade you consider
Good Very Good Some light will still reflect through the table of the stone, but you’ll notice a lack of sparkle compared to an Excellent cut diamond, and light is lost through the base and sides of the diamond
Fair Good The diamond will lose a significant amount of light through its base and sides, meaning it will appear dull and lacklustre
Poor Fair The majority of light will be lost through the diamond’s base and sides, meaning it will have next to no sparkle

Why is diamond cut important?

An Excellent diamond cut will enhance the appearance of other qualities of the stone, like the colour and clarity grade, and can even make a diamond appear larger. When a diamond is cut to an Excellent (GIA) or Ideal (IGI) grade, the polish and proportions of the stone will produce brilliance, fire and scintillation that work together in harmony, creating a charmingly sparkly diamond.

Brilliant cut vs step cut diamonds

The only diamond shape to receive a ‘cut grade’ are round brilliant cut diamonds. However, you should ensure any other diamond shape has Excellent symmetry and polish grade, or at the very least Very Good. ‘Brilliant cut’ diamonds are designed to exude the most sparkle, while step cuts feature larger, parallel facets that celebrate the clarity of the stone, with more subtle flashes of light

Comparing brilliant cut and step cut diamonds

Which diamond cut sparkles the most?

The most sparkly diamond cut is the round brilliant diamond, when graded triple Excellent (GIA) or Ideal (IGI). This means the diamond will have a the optimal cut, symmetry and polish grade. The round brilliant is the sparkliest diamond shape due to multiple lines of symmetry and 58 facets that work harmoniously to most efficiently reflect light within the diamond then out through its table. Round brilliant diamonds are typically the most expensive diamond cut, due to their unrivalled brilliance, popular demand, and because the shape requires the most diamond wastage during the cutting process.

What is the rarest diamond cut shape?

There are a few diamond shapes that are harder to come by than others. Asscher diamonds and radiant diamonds are rarer than most diamond cuts, because demand for the shapes is much lower than other shapes. Old mine cut diamonds, or old European diamonds, are also hard to come by - diamonds are no longer cut in this shape, so any in existence are typically inherited or belong to private collections, and very few are for sale.

Which diamond cut looks the biggest?

Brilliant cut diamonds can look bigger than step cut diamonds, as they emit more sparkle. Some brilliant cut shapes appear larger than others, thanks to an increased spread of the table; marquise, oval and pear diamonds appear larger than other diamond shapes of the same carat weight.

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