Diamond Color Grading

by Ms Nandita Ray

Diamond has Nitrogen atoms as the main impurity. The common concentration of this impurity is 0.1 %. If the nitrogen atoms are set in pairs then they have no affect on the diamond and make the diamond colorless. Such diamonds fall in the Type 1aA. If the nitrogen atoms are present in clusters they impart a brownish or yellowish hue to the diamond. These are clubbed in 1 aB.

Let us not be too particular, it is better to have old second hand diamonds than none at all', observed Mark Twain. Diamonds are worth more than their weight in gold. Diamonds are truly prized, even though they are crystalline carbons in their natural form. Diamonds are formed in the deep recesses of the earth by tremendous pressure and temperature. Diamonds come in many colors. Colorless diamonds are rare. There are diamonds of various colors. The most common color is yellow. This is because the chemical impurities and structural defects affect the color of the diamond. Chemically pure and structurally perfect diamonds allow light to pass through it and break up into the seven colors of the rainbow. Transparent and colorless diamonds are very expensive. Diamonds with their own natural color are unique and are called 'fancy diamonds'. Such diamonds are rare and very expensive.

A diamond's color can either enhance its value or decrease it. Most white diamonds with more yellow hue loses its price while a diamond with an intense pink, green, purple, blue or yellow becomes more valuable since they are rare. Out of all the colored diamonds, red diamonds are very rare. Diamonds may also have grey, orange brown or black color. However, most diamonds will always have a very slight hint of yellow.

How color grading is done?

Diamonds are scientifically divided into two classes and several sub class. This is based on the blemishes present in them, the nature of the blemishes and also how these blemishes absorb and reflect color. Diamond has Nitrogen atoms as the main impurity. The common concentration of this impurity is 0.1 %. If the nitrogen atoms are set in pairs then they have no affect on the diamond and make the diamond colorless. Such diamonds fall in the Type 1aA. If the nitrogen atoms are present in clusters they impart a brownish or yellowish hue to the diamond. These are clubbed in 1 aB. Ninety eight percent of the diamonds mined are 1a and rest is a mixture of 1aA and 1aB. The diamonds that fall in this category are mined in the Cape Province of South Africa. The rare Canary diamonds, which are a vogue with the Rappers, belong to the rare breed of diamonds which are type 1B. The Nitrogen atoms in such diamonds are spread through out the crystal and not present in groups. This gives the diamond a deep yellow or brown tint. Only 0.01 % of such diamonds are found. Synthetic diamonds which contain Nitrogen fall into the 1b group.

Type 1 diamonds have the capacity to absorb in the infrared and ultraviolet regions. They also have fluorescence characteristic. Type 2 diamonds have fewer Nitrogen atoms and can be colored pink, red or brown. These diamonds are rare. They are mostly mined in the Australian mines. Presence of Hydrogen gives the diamond a blue hue.

Color grading of diamonds is based on the color intensity scale as established by the GIA i.e. the Gemological Institute of America. There are other diamond grading agencies that have their own grading scale, the European Gemological Laboratory or the American Gemological society. But the accepted standard of grading is based on the GIA. It uses D to Z scale for grading the color of non fancy white diamonds. D is absolutely colorless while Z is yellow.

D E F - colorless

G H I J - near colorless

K L M - faint yellow or brown

N O P Q R - very light yellow or brown

S T U W X Y Z - definitely light yellow or brown color.

Diamonds that are at the top of the scale are also called 'high color' diamonds. Those that are placed at the bottom of the scale are called 'low color' diamonds. Diamonds that have a deeper color than Z are considered 'fancy diamonds'. Such diamonds are graded based on the characteristics of color present. This system of grading is similar to the grading of colored stones like Ruby, Emerald or Sapphire. Deeper the color the rarer it is. Colors such as deep purplish pink are very rare and thus most expensive.

How to test diamond color ?

There are two ways to test the color of a diamond. A stone can be compared and its color determined by using reference set or master set of stones. The best way to use this set is under natural light. The master set has stones graded according to the colors they exhibit. This set is made up of cubic zirconium. This way of determining the color is not always perfect. The best and decisive method to grade color is by using a color meter. This is an electronic instrument that is used to measure the optical characteristics of the stone and reveal the color. The Gram color meter which is calibrated according to GIA and AGS standards easily grades color and reduces the risk of incorrect deduction.

The diamond should be tested when it is loose. Setting affects the color as does the metal that has been used. The GIA's scale is the accepted standard of grading the color of diamonds. The European Gemological Laboratory has its own method. The table below indicates the system they use.

GIA	UK (traditional)	Color
D E F G	Finest white	colorless
H	White	near colorless
I	commercial white	very faint yellow
J K L	Top silver Cape	faint yellow or brown color
M N O P Q	Light Cape to Cape	very light yellow or brown
R	Cape to Dark Cape	definitely yellow or brown
S T U V W X Y Z	Dark Cape	definitely deep color

 
As clear the terminology varies but grading is similar. There is no standard scale for measuring colored diamonds. They are graded in two ways, the basic hue or tint of color and secondly the intensity of the color present. The basic hue will be pink, yellow, green, brown or purple. These two qualities determine the value of these so called 'fancy diamonds'. The deeper the color the more expensive the diamond even though they might be of the same size, shape and clarity. The GIA has 9 categories to grade color diamonds.

1 Faint

2 very light

3 light

4 fancy light

5 fancy

6 fancy dark

7 fancy intense

8 fancy deep

9 fancy vivid.

The GIA has some guide lines that need to be adhered to strictly while grading the color of a diamond.
1. The color of a diamond can only be compared to the color of the 'control or master set' of stones.
2. Graded diamonds must be loose stones.
3. The diamonds must be viewed with 10X loupe
4. Diamonds must be placed table down
5. A letter system from D to Z is used to identify the color
6. D grade is given to only rare and absolutely colorless diamonds.

The GIA diamond reports indicate whether a diamond changes its color under UV radiation. Diamonds with this quality can appear to change color under fluorescent lights. Diamond's that produce a blue color appear colorless under UV lights. Coatings are sometimes used to enhance the color of a diamond. Many other testing laboratories are
also including these specifications.