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A precious stone that is a variety of transparent corundum of any colour other than red (which is a ruby). The usual and preferable colour (‘Kashmir blue’) ranges from pale cornflower-blue to deep velvety blue; but less valuable varieties of corundum of other colours are included as sapphires, e.g. white, yellow, green, pink, purple, brown, and black. Any sapphire that is not blue is sometimes called a ‘fancy sapphire’. The cause of the variety of colours is uncertain, but the blue colour is probably due to traces of oxide of iron and titanium.
Some sapphires change colour in daylight from that in artificial light. Due to the Asiatic origin of the stones, the yellow variety is sometimes called ‘Oriental topaz’, the dark green ‘Oriental emerald’, and the purple ‘Oriental amethyst’; but these are misnomers that should be discarded. Sapphires are usually cut as a brilliant or mixed cut, but sometimes they are cut and strung as beads.
Sapphires show strong dichroism, often asterism (as in the case of the star sapphire when properly cut en cabochon), and occasionally feathers; they are extremely hard and have a vitreous lustre. Zones of different colours, or concentrations of one colour, are sometimes present in stone, as a result of which, and also owing to the dichroism, the quality of cut stones depends greatly on skilful cutting; faceting is done mainly in Sri Lanka (formerly Ceylon).
The finest stones came from India (Kashmir) since 1862 and have a rich blue colour that does not ‘bleed’ (i.e. change in different lights); the next finest blue sapphires (paler vanetles) came from Burma and some are still being found in Sri Lanka.
The colour of sapphires is sometimes altered by heat treatment (which dulls the colour) or irradiation (the effect of which fades).
Among the most famous or largest sapphires are the Bismarck sapphire, Lincol sapphire, Logan sapphire, Raspolie sapphire, and gem of The Jungle, and among those that show asterism are the Star of Asia and Star of India.