The classical idea of a diamond is the transparent, clear colour free from any chemical impurities or structural defects. However, in the real world, almost no diamond is absolutely perfect. Instead, the right impurity or defect in the crystal lattice can produce beautiful colours.

The rarest colour for a diamond might surprise you. First, let’s understand how diamonds become coloured.

Type I Diamonds

The chief impurity of type I diamonds is nitrogen atoms. When they’re paired, they don’t influence the colour of the diamond, but as they aggregate they can begin to create brown or yellow tints.

The more nitrogen, the more dispersed it becomes through out the crystal. This creates isolated concentrations that create brown or yellow tints. These are Type Ib diamonds.

Golden yellow canary diamonds make up only 0.1-percent of all naturally-occurring diamonds, but they aren’t the rarest.

If you have a good eye, you may notice that these diamonds appear slightly different than Type II diamonds. It’s true: Type I diamonds have different characteristics when it comes to fluorescence and absorption of visible light.

Type II Diamonds

These are diamonds that possess no nitrogen impurities – at least not at measurable scales. They absorb a different band of the spectrum and transmit in ultraviolet light. This is very unlike Type I diamonds.

Type IIa may be brown, pink, or red due to what’s called plastic deformation. This happens in the lattice at the crystals grow. These diamonds are rarer. They make up only 1.8-percent of diamonds, but are found disproportionately in Australia.

Type IIb diamonds are generally lighter blue because of the presence of boron. This also makes these diamonds good semiconductors.

Colours from Both Types

Some colours can result from either type of diamond – brown is much more likely from a Type I diamond just because they’re more numerous, but can also result from Type II diamonds. Light blue can result from Type IIb diamonds because of boron, but a darker grey-blue (such as in the Hope Diamond) can arise from Type I diamonds.

Green diamonds can occur in both type as well, and are the result of a diamond that’s been exposed to radiation. Black diamonds can result when graphite or a sulfide enters the crystal matrix. The look of these is particularly unique. White diamonds that are opaque instead of transparent can also occur. Purple diamonds often have high hydrogen content.

The Rarest Colour

Red diamonds remain the rarest, though they are not necessarily the most valued. When it comes to the rarest diamonds in the world, story matters as much as colour. This is why the Hope Diamond and Oppenheimer Blue are among the most expensive ever sold.

Thus, red is the rarest of diamond colours, but it doesn’t necessarily mean you have the rarest individual diamond.

Additionally, individual diamonds are graded on a range of other factors, including but not limited to clarity. Each diamond has its own unique personality and characteristics that make it different from the next. Yet if you’re looking for rarity, red is the order of the day.