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.

The traditional diamond is clear and colourless but diamonds also come
in over 300 colours. The diamond is the most “precious” of precious
stones and stands in a category of its own. The other precious stones also come in a myriad of colours and although there are three clear categories (rubies, sapphires and emeralds) they are often grouped collectively as “coloured stones.”

The concept of “precious” is measured on the basis on two criteria. Hardness and scarcity.          

A precious stone’s “hardness” is determined by reference to the “Hardness Scale” developed by German geological scientist Friedrich Mohs back in the 19th century. His simple scale rates material from one to ten. Diamonds are the hardest substance known to man so they measure a ten whilst talcum powder would be rated a one. Rubies and sapphires rate a nine and emeralds rate a seven. Any substance that rates less than seven is likely to be corroded by our environment, so the “hardness” of the stones in question gives them an enduring quality that makes them “precious.”

With respect to scarcity, it’s a simple case of limited supply leading to excess demand. Rarity makes the “heart grow fonder” and the stone becomes all the more “precious.” To unearth one carat of diamond you would have to mine 250 tonnes of minerals and only one in 10,000 of those diamonds will be coloured. The other precious stones also echo this “relative scarcity” to varying degrees.