Most diamonds in the world are slightly coloured with a faint trace of yellow or brown, although they can exist in almost any colour. The amount of colour a diamond possesses has a huge impact on its value. In increasing order of rarity, yellow diamond is followed by brown, colorless, then by blue, green, black, pink, orange, purple, and red. By far the most common fancy colour is yellow; in the year 2003, for example, 58% of coloured diamonds graded by the GIA were yellows, with all of the other colours together making up the balance.

Diamonds that are colourless or only slightly coloured yellow or brown are classified on the “normal color scale”, which is an alphabetical scale, running from D, which is totally colourless, to Z, which is noticeably coloured. The closer to D colour, the whiter and more valuable the diamond.

Coloured diamonds

Coloured diamonds are ranked according to their rarity and value. The causes of the colour is as follows:

  • Blue is caused by the presence of the element Boron, which changes the conductivity of the diamonds.
  • Red, Pink and strong brown are caused by crystal lattice defects, during the formation of the diamond.
  • Yellow is caused by trace amounts of nitrogen in the diamond.
  • Green is caused by exposure to natural radiation in the earth.
  • Black diamonds are not truly black, but instead contain numerous dark inclusions that make the stone seem black.

The colour of a diamond does not change over time; some treatments exist today to enhance the colour grade of a diamond, but treated diamonds are a lot less valuable than their natural counterparts!

Red Diamonds

Red diamonds are probably the rarest and most valuable diamonds of all in the world. True red diamonds are very scarce, and have long been sought after by gem collectors, jewellers and consumers. Strictly speaking, they are too rare for there to be a “market” for them, but whenever one comes up for sale they are highly valued. For example, a 2.26 carat purplish red diamond sold at auction in 2007 for 2.7 million dollars!

 

The colour system for fancy colour diamonds

There are two elements of a colour grade for a fancy colour diamond; together, they give a description of the diamonds appearance. The first is the colour description. Although the human eye can recognise miillions of colours, the GIA has 27 possible colour descriptions.

The colour wheel has basic colours, for example blue or yellow, and in-between colours, like greenish yellow or bluish purple.

The main colour is stated second, in capitals, the modifying colour is stated first in lower case. So gY would stand for greenish yellow, and yG would stand for yellowish green. For some colours they can be further modified to include the terms “slightly” or “strongly”, so “slpR” is “slightly purplish red”, or “vstgB” is very strongly greenish Blue”.

The colour is then quantified on a 9 step scale, ranging from faint to fancy vivid. In general, the closer to Fancy Vivid, the more valuable the diamond. This is useful for yellow diamonds, for example, as they occur in a wide range of intensities and depths of colour. As red diamonds are so rare, and diamonds that are red have a very tight range and depth of colour, the GIA has so far found only the description “Fancy” to be necessary.

Causes of Red in diamonds

The cause of red in diamonds is not completely understood. There is no reason to believe that it is caused by trace elements like nitrogen or boron. (These have a role to play in the colouration of yellow diamonds). The red colour in diamonds is believed to be caused by plastic deformation, which in the simplest terms is a “ripple” in the diamond crystal, caused by the intense pressure during the formation of the diamond, which makes the planes of the crystal displace slightly. Another potential cause is “gaps” in the diamond crystal, or to call it by its full name, “an atomic-level lattice defect”! It is possible that these two things combine together as well.

Rarity of red diamonds

In 2002, the GIA published a list of red diamonds that they had graded. (Some were excluded from the list due to client confidentiality.) There were only 15 diamonds in the list, 8 of which were below 1 carat in weight, and 14 of which were below 2 carats in weight. Only four were graded as “Fancy Red”, the others all had a modifier of the colour, for example “purplish red”. In addition, the Kazanjian Red, a 5.05 carat red, and 5.03ct De Young Red are large red diamonds, but the latter two are not mentioned in the GIA’s list. The 2.26carat red diamond mentioned in the first paragraph was not in the list either.

The largest, the Moussaieff Red, weighs 5.11carats, and is a Shield Shape Fancy Red diamond, last examined by the GIA in 1997. It was exhibited in the Smithsonian Museum in the summer of 2003, and is pictured at the top of this post.

In 1934 a five carat ruby red diamond was reportedly shown at the Chicago World’s Fair; it was valued at the time at $150,000. It is not the same diamond as the Moussaieff Red, as that diamond was only mined in the 1990s in Brazil. What happened to this stone, unfortunately, seems to be a bit of a puzzle! Could it be either of the two other large reds mentioned?…

The Kazanjian Red

One interesting red diamond with a good history is the Kazanjian Red. The 35 carat rough diamond was discovered in the 1920s in South Africa, and was initially sold for eight pounds. It ended up at a diamond cutter in Amsterdam, who spent seven months studying the diamond before starting to cut it. The end result was a stunning 5 carat emerald cut stone.

In 1944, it was placed in a safe in Arnhem. It was discovered by the Nazis, and sent to Germany. It was hidden in a salt mine near Berchtesgaden in Bavaria. After the war it was sold to Ernest Oppenheimer, and thence to a private collector. It vanished from public view until 2007, when it was bought by Douglas Kazanjian. It forms the centrepiece of the Kazanjian Charitable Foundation.

Yellow Diamonds

The chemical cause of yellow in a diamond

The chemistry of a pure diamond is very simple, and is what causes its amazing strength and purity of light. Carbon atoms form a rigid tetrahedral structure, which, when repeated over many billions of atoms is immensely stable and hard to break up.

These type of bonds are called covalent bonds. Covalent bonds are the most stable type of chemical bonds. To cause a substance to emit light, you need to give the electrons enough energy to “move” them from this very stable state (called the valence band) to a more energetic state (called the conduction band). With pure carbon bonds, seen on the left in the diagram below, this “energy gap” is 5.4 electronvolts (eV), more than the energy in the visible light range; thus the diamond does not absorb light and will appear colourless. (One stray nitrogen atom lowers the gap to 4eV, still not enough to cause the diamond to appear coloured)

Tiny amounts of nitrogen cause this energy gap to narrow. Only a few atoms of nitrogen per million atoms of carbon can narrow this gap to as little as 2.2eV, as on the example on the right in the image below. This narrower gap means that the diamond will now absorb some light, and will appear yellow.

Effect of nitrogen in a diamond - Douma, M., curator. (2008). The Band Structure of Blue and Yellow Diamonds. In Cause of Color. Retrieved March 26, 2011, from http://www.webexhibits.org/causesofcolor/11A0.htm

Effect of nitrogen in a diamond – Douma, M., curator. (2008). The Band Structure of Blue and Yellow Diamonds. In Cause of Color. Retrieved March 26, 2011, from http://www.webexhibits.org/causesofcolor/11A0.htm

 

Where do Yellow Diamonds Come From?

Historically, yellow diamonds were very scarce; some are documented by Tavernier, the famous explorer and merchant who discovered many of the world’s most famous diamonds in India in the 17th century. He makes reference to a 137 carat yellow diamond known variously as the “Florentine” or the “Grand Duke of Tuscany”. However, stones such as those were very much the exception until the 1860s, with the advent of mining in South Africa. As most of them came from Cape Province, these diamonds are now known informally as “Cape” colour. Today, most yellow diamonds still come from there.

One of the most famous yellow diamonds in the world is the 128 carat Tiffany diamond, normally on display in their store on 5th Avenue, New York.

A recent arrival in stock is this interesting yellow diamond cluster ring, set with a 0.82 carat fancy yellow centre stone. Of note is that it is also of good clarity and cut, a rare combination. It is in a platinum mount, with pavé set diamonds surrounding.

Yellow Diamond

Yellow Diamond

Some famous celebrities who have had yellow engagement rings include the rather marvellous Heidi Klum, who had a fantastic cushion cut, Holly Madison and Rebecca Romijn. Interestingly, most of the large yellow diamonds one will see are cushion cuts, or modified princess cuts, and this is not a coincidence – generally, to enhance the depth and sensation of colour, the extra depth of those shapes gives a much more pleasing effect than a round stone will.

 

Blue Diamonds

Blue Diamonds are among the rarest and most enchanting of all coloured stones.

To understand why they are so rare, we need to look at the chemistry of diamonds. 98% of all diamonds are what as know as Type I (“type one”). Type I diamonds contain trace amounts of nitrogen. The other 2% of all diamonds are called “Type II” (“type two”).

hope

Image copyright Smithsonian Institute

There is a further division of Type II diamonds, into Type IIa and Type IIb.

Type IIa are almost totally chemically pure, containing only carbon. Type IIb contain minute traces of boron.

Type IIb is the category in which we find almost all blue diamonds. Unlike the other categories, Type two diamonds are electrical semiconductors.

The boron present in Type IIb diamonds can in some cases make a diamond blue. The mechanism is not fully understood, but we do know that almost every blue diamond in the world is Type IIb. However, the chemical rarity of these diamonds means that overall, less than 0.0001% of the diamonds in the world are blue diamonds.

The colour of blue diamonds runs from very pale blue, to a rich, deep intense blue. The deeper and richer the blue the more desirable the stone is.

It is possible to colour diamonds artifically nowadays, however these diamonds are much less valuable than their natural counterparts. The majority of blue diamonds for sale that one will see are either synthetic/simulant or are natural stones that have been artificially enhanced. Such enhancement may involve irradiation and/or heat treatment.

In October 2007, a 6 carat natural blue diamond broke all previous records, becoming the most expensive diamond (per carat) ever sold at publin auction. It sold for almost 8 million dollars, or 1.3 million dollars per carat. It is pictured below. What made it so valuable was the richness of blue, and the fact that it was evenly spread across the whole of the diamond.

Any diamond with even a slight amount of blue colour is classified as a blue diamond. Many have a grey modifier, that is to say the blue is “mixed” slightly with grey. The rarity and value of a blue increases dramatically the stronger and purer the saturation of blue gets and the less grey mixed into the blue.

The most important factor in evaluating a fancy color diamond is the richness and beauty of the color. The GIA grading scale for blue diamonds ranges from “Faint” and “Very Light,” to “Fancy Deep” and “Fancy Vivid” where the saturation and intensity of the color just reaches out and grabs you. Other considerations, including clarity and carat, are of much less importance than colour when discussing coloured stones.

The most famous blue diamond is the world is the Hope Diamond. A wonderful 45 carat rich blue diamond, currently in the Smithsonian Institue in Washington. Legend has it that it is cursed, though that is a subject of another blog posting some day!!

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