This week’s What’s on the van? comes from Monica Price, Assistant Curator of the Museum’s Mineralogical Collections.
What is that fine large blue gemstone on the van? Did you guess it was a sapphire? You were absolutely right; it is after all, the most famous of all blue gemstones. But did you know that sapphire is not always blue? It can be yellow, green, purple, pink, even colourless!
Sapphire is the name given to nearly all the coloured gem varieties of corundum, a mineral composed of aluminium oxide. Corundum is one of the hardest of all minerals and is used in industry as an abrasive. It is found in various aluminium-rich igneous and metamorphic rocks and it is so hard that it resists breakdown when the rocks undergo erosion by rivers and streams. Instead it accumulates in river gravels, which can be mined for gem quality crystals.
Most corundum is rather drab, not very exciting at all. Only the best coloured and least flawed crystals are suitable for cutting as gemstones. The colour in blue sapphire is due to a tiny amount of titanium and iron. When corundum contains a tiny amount of chromium instead, it is a beautiful shade of red, and it has a name of its own – ruby. Ruby and sapphire are simply different coloured varieties of exactly the same mineral!