When we talk about diamonds there are many words and phrases that may be unfamiliar to those who are not jewellers or gemmologists. A good number of these refer to the various parts of a gemstone and it can be useful to become familiar with these if you are planning to purchase a diamond. Familiarity with the anatomy of a diamond can really help with understanding why some stones sparkle more than others and what to look for when making your choice. In this blog we will provide some definitions of commonly heard terms and some of the reasons these parts of the gemstone are important.
Typically, cut diamonds are cut with flat surfaces in triangular and kite shapes across the whole of the stone. These are facets and they reflect the light that hits the surface of the diamond; their smoothness and flatness is part of the reason why diamonds have such bright lustre. Diamonds are incredibly hard and can take a high polish superbly, creating an unrivalled lustre known as adamantine (diamond-like).
The widest part of the diamond is called the girdle. The domed part above this is the crown while the pointed part below is the pavilion. The relationships between these areas are very important, the angles and relative sizes making the difference between a diamond that can return lots of light and sparkle and one that will look dull and lifeless.
The crown consists of two rows of triangular facets with a row of kite-shaped facets between them. The largest facet right on the top of the crown is the table and on a round brilliant cut this will be octagonal in shape. This is our window into the diamond, allowing us to see inside it and experience all the sparkle and brilliance that the stone has to offer. The ideal size of the table can vary depending on the shape of the stone, but a balance is normally required as tables that are too small or large can impact upon the ability of the diamond to display the sparkle and fire that make the stone so desirable. If the table is small then the dome of the crown may be quite steep, while a larger table may create a very shallow crown. The diamond below has a large table and resulting shallow crown.
The diamond below is an old cut stone with a very steep crown. This is quite typical of old diamonds cut before the advent of the modern round brilliant cut.
Fire refers to the multicoloured flashes you see as a diamond catches the light. This happens when the stone splits light into its separate wavelengths (exactly as a prism creates a rainbow when light passes through it). A smaller table creating steeper angles in the crown increases fire but diminishes brilliance (the reflections of white light returned by the diamond). Too large a table has the opposite effect. When determining the perfect proportions for a diamond to create the best balance of bright white light and multicoloured twinkle, the diamond cutter should typically choose the ‘Goldilocks’ balance to release the most beauty from the stone.
The pavilion is particularly important in determining how much bright white light - brilliance - the diamond returns to the viewer. It consists of elongated triangular facets that stretch down towards the base of the diamond. As the round brilliant cut developed over time, these facets became longer and added to the scintillating pattern of sparkle you see when you view the stone. Where they meet at the base of the stone, the diamond may or may not have a culet, which is a small additional facet parallel to the table. Most diamonds today have a pointed pavilion, but older stones can sometimes have a culet large enough to be seen with the eye when you look through the table. The dark circle in the centre of the diamond below is the culet. This won't be seen on most modern stones.
If the pavilion is too shallow the stone will lack brilliance and you may see a ‘fish-eye’ effect. This is a reflection of the girdle seen through the table and causes the diamond to look ‘flat’. However, if the pavilion is too deep the stone will appear dark as light will not be reflected back to the viewer. This is sometimes known as a ‘nail-head’ effect.
This brings us back to the girdle, typically a quite narrow surface that connects the upper and lower parts of the diamond. Diamonds are the hardest natural material we know of and until the development of lasers, they could only be cut or polished at all because certain directions in their crystal structure are harder than others; only a diamond could cut another diamond (the harder direction of one cutting the softer direction of another). Because of the complexities of these directions of hardness, for a long time the only way to create a round shape was to grind one diamond against another. This process is called bruting and it created a girdle that had a characteristic 'frosty' appearance.
Nowadays, girdles may also be polished or faceted. Polished girdles are more likely to be seen on some fancy shapes such as princess or emerald cuts, where each edge essentially becomes a single facet. On rounds and other shapes with rounded outlines, the girdle is usually faceted. A whole row of tiny rectangular facets ensures the girdle adds lustre and plays its part in the overall attractiveness of the stone, rather than simply being practical. However, the practicality of the girdle is important. It offers protection to the diamond at a point where it is particularly exposed. In very old diamonds it is common that the edge of the stone can appear very chipped and damaged because there was either no girdle or it was too thin and the edge where the upper and lower parts of the diamond met received many impacts over time.
Conversely, a girdle that is too thick can make the diamond chunky and look smaller than its carat weight would suggest (meaning you are paying for extra weight that you aren’t getting the benefit of seeing). A very thick girdle can also sometimes be seen reflected through the table facet and disrupt the brilliance and sparkle that you would like to see. This is especially unattractive where the girdle is bruted and the reflection of the roughened texture causes the diamond to appear cloudy or dirty. If the girdle is too thick it can also make the diamond difficult to set.
The girdle can be quite easily spotted on the image below (a lab grown pink diamond) - look for the darker horizontal line around the middle of the visible area of the stone.
When round brilliant cut diamonds are graded for their proportions (the ‘Cut’ grade on a laboratory report) the grader takes into consideration all these factors to build an overall picture of how well the stone is cut. Subtle deviations from what is considered ideal can prevent the stone from receiving an ‘Excellent’ grade and looking as beautiful as it should do. For this reason, we generally regard the Cut grade as being just as or even more important that the colour and clarity. Speak to one of our gemmologists and diamond graders to learn more.