Diamonds might purportedly be ‘a girl’s best friend’, but they’re also quite interesting from a chemical perspective. You could be forgiven for thinking that there’s not a whole lot to them; after all, they’re simply one of the possible forms of carbon, formed at high pressure beneath the Earth’s surface. However, there are a number of factors that can affect their appearance, and several of these are rooted in chemistry.
If you’ve never had the occasion to purchase a diamond ring, you might be unaware of the system diamond manufacturers and sellers use to classify diamonds. This system is referred to as the four Cs of diamond quality, the eponymous ‘Cs’ being cut, carat, colour, and clarity. These different attributes are usually given a form of shorthand to communicate a particular diamond’s characteristics, so trying to purchase a diamond-containing ring quickly becomes an exercise in cracking a cryptic code that relates the diamond’s quality.
The first of the Cs, the diamond’s cut, has the least to do with chemistry – but does have links with physics. The cut of the diamond is important in order to obtain the best sparkle from the gem. The ideal cut allows for total internal reflection of the light hitting the diamond, reflecting it back out of its face; if the cut of the gem is too shallow, or too deep, then this internal reflection is disrupted, and the stone appears less brilliant.
The diamond carat refers to the mass of the diamond. It’s a unit that’s used for other gemstones too; 1 carat is equal to a mass of 200 milligrams, or 0.2 grams. Typical diameters for a range of different carat weights are given in the graphic, though these can vary depending on the cut of the diamond. Generally, higher carat diamonds are predictably higher in cost, though some of the other ‘4 Cs’ can also have an influence.
It’s when we get to our remaining factors, colour and clarity, that the chemistry comes in. The colour of diamonds is graded on a scale that runs from D to Z – apparently, it starts at D rather than A to avoid conflict with other systems that were in use when this particular system was introduced. D represents a completely colourless diamond, with an increasing yellow hue as you move towards Z. This colouration is derived from impurities in the diamond’s structure. As well as this colour classification used by jewellers, diamonds can also be placed in one of four major categories which are perhaps more useful to us when discussing their chemistry. These are type Ia, type Ib, type IIa and type IIb.
Type I diamonds contain nitrogen impurities. For type Ia diamonds, which make up 98% of all natural diamonds, this is in the range of 0 to 0.3% nitrogen, which is present in the crystal in clusters. Type Ia diamonds have a colour ranging from near-colourless to yellow. Why though, does the inclusion of a small number of nitrogen atoms change the colour of diamonds in this way?
To explain this, we have to talk about band gaps. A band gap is the energy gap between the energy level of the outermost electrons of a material, and the next unoccupied energy level above this level. In diamond, this energy gap is pretty big. What this means is that when visible light photons hit the diamond, they don’t have enough energy to allow electrons to jump up from the highest energy level and across the band gap. Therefore, none of this light is absorbed by the diamond, and it appears colourless as the entire spectrum of visible light is transmitted.
However, when nitrogen atoms are present in diamond, a donor energy level is created, at a higher energy than diamond’s electrons are usually found. The band gap between this donor level and the unoccupied levels is now smaller, and so the absorption of some of the visible light spectrum provides enough energy for electrons to bridge this gap. As some of the spectrum has been absorbed, the diamond does not transmit all of the visible light that hits it, leading to the yellow colouration.
If the nitrogen impurities are more spread out than in the clustered type Ia diamonds, then we have ourselves a type Ib diamond. These are rarer than type Ia, making up just 0.1% of all naturally occurring diamonds. Their yellow colouration is also more intense. Synthetic high temperature, high pressure diamonds also tend to be of this type.
Type II diamonds differ from type I in that they contain no measurable nitrogen impurities. Type IIa, which make up 1-2% of all naturally occurring diamonds, are often completely colourless, though structural anomalies in the tetrahedral structure of the carbon atoms can themselves produce a range of colours. Synthetic diamonds can be produced using chemical vapour deposition (CVD), and these diamonds are usually type IIa. Type IIb diamonds may not contain nitrogen impurities but they do contain impurities of a different element: boron. These diamonds are relatively rare, making up only 0.1% of all naturally occurring diamonds. Absorption of visible light as a result of these impurities leads to the diamond having a prized blue colouration.
The final C, clarity, also has links with the impurities found in diamonds. These impurities can manifest themselves as cloudiness or imperfections in the diamond, which can vary in how noticeable they are. Part of a diamond’s ‘code’ refers to these imperfections. Diamonds which contain no imperfections are designated ‘flawless’, with those containing only minor surface blemishes designated ‘internally flawless’. The scale then continues with increasing numbers of imperfections; through ‘very very slightly included’, ‘very slightly included’, and ‘slightly included’, to the lowest grade, ‘included’. At this final stage, imperfections (or inclusions) are likely to be visible to the naked eye.
Moving away from diamonds, we can also examine the ring’s chemical composition. Today, you can choose from a range of metals: gold, silver, platinum, and more. Gold itself is actually too soft to be used in its pure form, and so it’s commonly alloyed with other metals. White gold is an alloy of gold with palladium, nickel, and zinc, which is often plated with rhodium to increase its durability. The more stereotypical yellow gold is still an alloy, in this case with copper and silver. Silver itself is commonly alloyed as well; if you have a silver ring, it’s likely also got copper and possibly some other trace metals present.
Away from the traditional metal choices, and other metals have been putting in appearances. Platinum is now a popular choice due to its similarity in appearance to white gold, but greater durability; palladium is being increasingly used for similar reasons. Another metal used in rings, though perhaps more for wedding rings than engagement rings, is tungsten in the form of tungsten carbide.
(Finally, in case you’re wondering what prompted this decidedly non-festive post… it’s exactly what you suspect, and she said yes! In fact, the pictured ring is THE ring.)
References & Further Reading
- Band structure of blue and yellow diamonds – Web Exhibits
- The 4 Cs – GIA
- Theory of Impurities in Diamond – P R Briddon & R Jones