Join gemmologist and Head of Jewellery in Scotland, Ruth Davis FGA DGA, as she introduces us to the fascinating and colourful world of sapphires with her specialist selection from our forthcoming Jewellery, Watches & Silver auction on 08-09 December.
Many of us associate sapphires with those wonderful rich Royal Blue gemstones (famous for other ‘Royal’ reasons) or perhaps those soft, gentle cornflower blue examples, that we are all so familiar with. Some of us may even be aware that they come in a whole rainbow of colours, excluding importantly, red.
Sapphire is a variety of the gem mineral family called Corundum; the other variety of Corundum is ruby. The two gemstones are remarkably similar, sharing many of the same physical and optical properties; importantly the red variety of corundum is always ruby; while the remaining colours – blue, green, pink, purple, yellow, orange etc. – are sapphires. When does a pink sapphire become a pinkish red ruby I hear you ask – well that is an ongoing source of debate, and you will often hear people quip ‘it depends on if you are buying or selling.’
While the whole gamut of colours available in sapphires may be a revelation to some, how many of you I wonder know about the rarest of sapphire examples, the colour-change sapphire. This chameleon of the corundum family changes colour depending on the light in which it is viewed, generally under daylight or an equivalent they appear blue to violet, and under incandescent light sources (such as light bulbs) they present a violet-purple to reddish-purple. This is caused by the way in which the chemical elements within the gemstone interact with the light source, certain elements absorb specific wavelengths from the different light sources, the presence of chromium and vanadium in the sapphire are most often responsible for this colour change.
The most important factor in the ‘quality’ of these gemstones is how pronounced the colour-change is, some stones exhibit a weak colour change and it can often be barely noticeable, while in stones with a ‘moderate’ or ‘strong’ colour-change the difference in colour becomes more pronounced. Lot 110 in our forthcoming Jewellery, Silver & Watches auction is a wonderful example of this rare sapphire variety, of Sri Lankan origin and showing no evidence of heat treatment (often carried out to enhance colour) it shows a purplish-blue colour in daylight, and purple in incandescent light; a worthy additional to any gemmologist or jewellery collectors’ collection.
Viewing by appt from 28 Nov to 07 Dec (closed 04-06 Dec)