Diamonds that are intense yellow or brown, or any other colour are called fancy-coloured diamonds or fancies, and are graded as Z+ on the GIA colour scale. All coloured diamonds are rare. Out of 10,000 carats of diamonds mined, only one carat will turn out to be fancy-coloured. Coloured diamonds can be gray, blue, yellow, orange, red, green, pink (the most popular), purple, brown and black - virtually every colour of the rainbow. The rarest coloured diamonds are red, blue, green, orange and pink. Coloured diamonds occur from chemical impurities and / or structural defects in the diamond’s crystal lattice.
The yellow tint that occurs in diamonds is caused by nitrogen mixed into the carbon lattice. The more intense the yellow colour, the higher the concentration of nitrogen. Some of the finest yellow diamonds come from South Africa.
Boron mixed into the carbon lattice forms the blue colour in a diamond. The intensity of the blue colour is determined by the concentration of boron. Blue diamonds tend to be very clean on the inside. Their grades are usually VS or higher. Some of the rarest and finest blue diamonds come from Russia and South Africa.
The green diamond is caused by radiation. When a diamond is formed in the earth near a uranium or radium deposit, it will develop a green colour over many millions of years. Green diamonds are very rare and most are found in Brazil.
Pink and red diamonds are extremely rare. Red diamonds are the same as pink except they have every strong tone and saturation. The colour in pink and red diamonds is caused by graining within the diamond. The vast majority of pink and red diamonds come from Australia.
Purple diamonds are a combination of pink and blue diamonds.
Orange diamonds are a combination of yellow and a little bit of brown. A very intense orange is extremely rare in nature.