Fancy Coloured Diamonds
Colourless diamonds are rare enough, but naturally-occurring fancy coloured diamonds are rarer still - which explains why they often command the highest prices at auction. Diamonds can come in all sorts of colours, from vivid blues to fiery oranges - but how do they get these spectacular colours in the first place?
Fancy coloured diamonds ('fancy' denotes that the diamond acquired its colour naturally, rather than being artificially treated) occur in nature for several reasons. A colourless or "white" diamond is made of 100% carbon. When another element gets into the carbon chain, it can add a colour - for instance, nitrogen causes yellow, brown and pink hues, while boron produces blue or blue-grey. Hydrogen can cause diamonds to become red, violet, blue or green.
Another way for the stones to acquire colour is via unusually intense pressure or heat during the compression stage that gives birth to diamonds: this can lead to red, pink or purple diamonds. Naturally-occurring radiation can also affect the colour, making diamonds blue or green - mines in certain parts of the world have a greater chance of unearthing these.
One particularly interesting variety is carbonado - the "black diamond". These diamonds have a different crystalline structure to regular diamonds that makes them absorb light rather than reflecting it. They have traditionally been associated with bad luck and sold at lower prices than regular diamonds, although recently their distinctiveness has seen them gain in popularity.
What makes carbonado fascinating is that scientists can't agree on how it is formed: some say it's made when meteorites strike the Earth, others by direct conversion of carbon in the planet's interior, while still others say it's formed inside dying stars, and is blasted to Earth via a supernova. To date, black diamonds have only been found in Central Africa and Brazil.
The presence of a colour might seem like it makes the diamond technically less 'pure', but the rarity and sheer beauty of these stones makes them highly sought-after. To illustrate just how rare they are, a mine producing 35 million carats (seven metric tonnes) a year would consider itself lucky if it found four or five of the rarest-coloured diamonds in that time!