Historically the term ‘precious stone’ has referred to only four types of stones, diamond, sapphire, emerald, and ruby. In many cases this is due to the fact that the technology and knowledge needed to tell different kinds of stones of the same colour apart did not yet exist, and so often the stones in many of our extant pieces, especially pieces preserved through royal lineage, are often referred to as ‘sapphire,’ ‘diamond,’ ‘ruby,’ or ’emerald,’ based solely in their colour. This of course means that we often find garnets referred to in historical texts as rubies, spinels referred to as sapphire, and tsavorite referred to as emerald, to name but a few examples.

In the modern era we posess not only the knowledge but also the equipment to be able to tell these different kinds of minerals apart with precision, but the remnant of their historical classifications remain in the way we refer to the two different groups; precious, and semi-precious, though these classifications can often be misleading since a good quality rare gemstone classified as semi-precious can far exceed the lower grades of precious stones in value. For this reason the trade has started to discard this way of classification, because of the false perception of value it creates in the eye of the public.

In the grading and valuation of colored gemstones (excluding diamonds) the following factors are generally considered, each of which will influence the price. This list is a simplification of the terms for easy reference.

• Hue – where the colour falls on the spectrum (eg. greenish-blue versus cornflower blue)
• Saturation – how intense the colour is and how evenly it is dispersed (does it look watered down or deep and full? Is the colour deeper on one side of the gem or is the gem an even colour all the way through?)
• Tone – does the colour look bright or muted? (eg. bright pink versus dusty pink).
• Clarity – are there any cracks or blemishes on the inside of the stone?
• Origin – where was it mined?

Sapphire & Ruby

Sapphire and ruby are in fact two different colour versions of the same mineral, corundum (Al2O3 – aluminium oxide). When the red colour is caused by the presence of chromium in the crystal structure, the gem is classified as ruby, all other variants are classified as sapphire, even the red ones that do not contain chromium. Corundum is available in almost every colour in the visible spectrum, and prices range from just a few dollar per carat to thousands of dollars per carat, depending on the colour, hue and saturation or colour intensity, the clarity of the gemstone, how well it is cut, the caratage and even the mine of origin.