About Diamond Certificates
The two biggest laboratories in the world are GIA and EGL. Both of these are reputable with the main difference being that GIA applies a stricter grading system whereas the EGL approach is ‘softer’.
This may result in the same stone obtaining a different grading from the two laboratories. GIA might grade the colour of a stone as ‘H’ while EGL might grade it as ‘F/G’ – GIA might grade the clarity of a stone as ‘SI2’ while EGL might grade it as ‘VS2/SI1’.
Both these laboratories grade stones according to their own master set of colour stones and clarity rules. This difference is also reflected in the price of the stone. A stone with a softer grading will carry a bigger discount than a stone with a stricter grading, which equals out the playing field.
EGL and GIA grades colour according to the international scale of D to Z. Every letter on the scale represents a range of tone and saturation.
Two diamonds belonging to the same colour range can actually differ in their depth of colour. Since brightness and fire, which are the result of good proportions, can affect a diamond’s colour, it is important to grade a diamond not only face down but also face up, as diamonds are typically set and worn face up.
In the face up position, diamonds with moderate or distinct fluorescence have a better colour than less fluorescent ones.
Colour and Clarity
Colour and clarity grading is an opinion of humans grading the stones and not an exact science. The same lab might at different times (with different employees) grade the same stone differently.
However, on the most important part of the grading report, both laboratories are exactly the same. The proportions of the diamond i.e. the table width, total depth, crown and pavilion angles etc. are an exact science and are measured and calculated using sophisticated measuring instruments.
These proportions of the diamond determine the brilliance of the stone and therefore should be ideal. An ideal cut ‘H’ is a much better stone to buy than a poorly cut ‘F’.
This is the most common style of cutting both diamonds and coloured stones. The standard round brilliant consists of 57 facets; 1 table, 8 bezel facets, 8 star facets, 16 upper-girdle facets on the crown; 8 pavilion facets, 16 lower girdle facets; and usually a culet on the pavilion. Modifications of the round brilliant include such fancy shapes as the marquise, half moon, pear shape and many others.
The Princess is a relatively new shape and generally has 70 to 76 facets (no culet). Normally it is close to a square shape (+ or – 10%), but may come in elongated versions. Watch out for girdles, which are extremely thin and thus prone to chipping. Typical length-to-width ratio: 1 to 1.00-1.10.
The Pear is a variation of the Brilliant cut, combining the Round and Marquise cuts, with 58 facets to only 56 facets (when the pavilion facets at the head and tail are eliminated). Shoulders should have a gently but distinctly rounded arch. Common length-to-width ratio: 1 to 1.50-1.75.
The Oval is a brilliant style of cutting very similar to a Round except it is elliptical, and was invented by Lazare Kaplan in the early 1960s.
Oval brilliant usually has 56 or 57 facets. Beware of uneven or high shoulders (they should have a gently but distinctly rounded arch). Typical length-to-width ratio: 1 to 1.30-1.65.
This shape has a boat shaped girdle with 57 facets. The shape and placement of the facets is of the brilliant type. The name ‘Marquise’ came from a legend of the Marquise of Pompadour that the Sun King wanted a Diamond to be polished into the shape of the mouth of the Marquise.
Look for uneven ‘wings’ or undefined points. Typical length-to-width ratio: 1 to 1.75-2.25.
The Emerald is a form of step cutting. It is usually rectangular but sometimes may be square, in which case it is known as a square emerald cut. It has rows (steps) of elongated facets on the crown and pavilion, parallel to the girdle, and with corner facets. The number of rows of elongated facets may vary, although the usual number is three on the crown and three on the pavilion
Inclusions are slightly more visible in ‘step-cut’ shapes relative to ‘brilliant’ styles. Look for too narrow or missing corners. The bevelled corners protect the stone and make it easier to set. Typical length-to-width ratio: 1 to 1.30-1.70.
This is a rectangular or square stone with cut corners. The original patented cut has 70 facets but it is readily available in modified versions with 62 to 70 facets. Typical length-to-width ratio: 1 to 1.20-1.50 for the rectangular stones.
Look for uneven or flat ‘wings’ or too shallow cleft. Typical length-to-width ratio: 1 to 0.90-1.10.
Trillion is a popular choice for side-diamonds to enhance a centre diamond. Typical length-to-width ratio: 1 to 0.90-1.10.
The Cushion evolved from the ‘Old Mine Cut’ that was developed before the turn of the century. It is a square to rectangular cut with rounded corners and 58 facets and is characterised by large facets, depth, and an open cutlet (the tip on the bottom of the diamond).
What determines the value of a diamond?
There are four main factors that determine the value of a diamond. They are referred to as the Four C’s:
- Carat Weight (size)
A diamond’s weight is expressed in carats. Carat weight is the easiest of the Four C’s to determine. To get the exact weight however, the diamond must be loose. One carat is divided into 100 ‘points’ so that a diamond of 50 points is described as a half carat in size, or 0.50 carat. Within common weight ranges, there is little or no variation in per-carat price.
The cut of a diamond is also referred to as its ‘facets’ and relates to its proportion. Many cutters choose to sacrifice some of the diamond’s beauty to produce a stone that is a larger carat weight.
Cut, more than any other quality aspect, gives the diamond its sparkle. A diamond gets its brilliance and scintillation by the cutting and polishing of its facets, allowing the maximum amount of light that enters through its top to be reflected and dispersed back through the top.
With proper cutting the light passes through the top, bounces off the sides, and then travels back out the top, giving the diamond optimum brilliance. If the diamond is cut too deep, light passes through the side of the diamond. If the diamond is cut too shallow, light passes through the bottom of the diamond, also inhibiting maximum brilliance.
Two popular overall proportion indicators are Total Depth Percentage (D%) and Table Percentage (T%). D% is the diamond’s depth expressed as a percentage of its width (average diameter for rounds). T% is the diamond’s ‘table’ width expressed as a percentage of its overall width (diameters for rounds).
Round diamonds with cutting proportions within the range generally considered attractive, have depths from 55 to 63 percent; the table size of most round diamonds lies between 56 and 64 percent. With non-round shapes (‘fancy’ shapes), much greater proportion variations are encountered. In most fancy shapes, higher D% and T% are more common and are dependent on width to length ratios. See Round Facet Diagram below.
Diamonds come naturally in almost every colour of the rainbow. However, most people prefer diamonds in the white range. Colourless diamonds are more valuable, because they are rare. The lack of colour, or whiteness, in a diamond allows the light to pass effortlessly through the stone and disperse that beauty back to the observer.
The colour grading scale established by the Gemological Institute of America (GIA) varies from D (totally colourless) to Z (light yellow). D through F are virtually colourless. G, H and I diamonds appear colourless when mounted. J, K and L diamonds look very nearly colourless, but you can see some colour in larger stones.
Beyond M, most people can see colour pretty easily. The colour of the metal in a mounting can either mask or enhance the diamond colour. Yellow gold makes slightly yellow or brown diamonds appear more colourless. White mounting (gold or platinum) makes the colour more perceptible. See Colour Diagram below.
Clarity describes the clearness or purity of a diamond, and is determined by the number, size, nature, and location of the internal (inclusions) and external (blemishes) imperfections.
Are specific treatments, performed on natural diamonds (usually those already cut and polished into gems), which are designed to improve their visual gemological characteristics, and therefore the value, of the diamond in one or more ways. These include clarity treatments such as laser drilling to remove black carbon inclusions, fracture filling to make small internal cracks less visible, color irradiation and annealing treatments to make yellow and brown diamonds a vibrant fancy color such as vivid yellow, blue, or pink.
Clarity and color enhanced diamonds sell at lower price points when compared to similar, untreated diamonds. This is due to the fact that enhanced diamonds are originally lower quality before the enhancement is performed, and therefore are priced at a substandard level. After enhancement, the diamonds may visually appear as good as their non-enhanced counterparts.
Fluorescence is not formally a colour grading term. Many diamonds glow when exposed to light that contains relatively high amounts of ultraviolet. This is due to a natural interaction between the light’s energy and the atoms in the diamond.
Some diamonds (about 10%) fluoresce strongly enough so as to be somewhat noticeable in regular (incandescent) light. Strong fluorescence in colourless to very near colourless grades (D through G) can sometimes give the diamond a hazy appearance. Generally, for very light yellow colour diamonds, fluorescence is considered to be beneficial since it makes the diamond appear whiter. The beauty of any diamond that exhibits ‘faint’ fluorescence is not adversely affected in any way.
How to estimate the weight of a diamond
|Round Cut||Diameter x Diameter x Depth x .0061|
|Oval Cut||Length x Width x Depth x .0062|
|Princess Cut||Length x Width x Depth x .0083|
|Heart Cut||Length x Width x Depth x .0059|
|Triangle Cut||Length x Width x Depth x .0057|
|Emerald Cut||Length x Width x Depth x Factor:|
(Length to width ratio)
|Marquise Cut||Length x Width x Depth x Factor:|
(Length to width ratio)
|Pear Cut||Length x Width x Depth x Factor:|
(Length to width ratio)