There are four stones that are classified as precious, they are diamonds, rubies, sapphires and emeralds. When considering these precious stones it is important to recognise the clear distinction between coloured diamonds and coloured gems, as each of these stones has its own specific character both in terms of chemical makeup and physical properties.

Diamonds are comprised of carbon and emeralds are made of beryllium whilst sapphires and rubies share a common constituent in alumina. The differences amongst precious stones are further accentuated by the trace elements that alter their composition over centuries of evolution.