Posts Tagged Corundum
The Corundum Conundrum – Part 1
Posted by Michael Wall Bepsoke in The Gems of Information on September 30, 2012
The title would make a marvellous Robert Ludlum book don’t you think? Yes? No, ok I’ll stick to the day job.
What is Corundum?
Corundum is a Mineral. If it’s Red, it’s a Ruby, if it happens to be a specific Pink-Orange colour it may well be Padparadscha.
For the purpose of this blog post I will be referring to all other colours of Corundum. The Greens, the Pinks, the Whites, and the Blues.
The Corundum otherwise known as, Sapphire.
Sapphires are most readily recognised as being Blue.
The blue Sapphire has been compared to Hyacinths and the Heavens.
The ancient Persians believed the Earth sat on Sapphire, and the stones colour was reflected in the skies. And who could blame them. The silky powder hue of a Cornflower Blue Sapphire is certainly evocative of hazy summer skies.
Pre eighteenth Century it was very easy to distinguish Sapphire, as blue stones were categorised as ‘hyacinth’. In fact it was a lot simpler as Stones were classified by colour alone. Anything Green was Emerald, anything Blue, Sapphire and so on.
Pliny the Roman encyclopaedist understood that properties other than colour were essential in identification of the Gem; however he was under a distinct disadvantage. He did not know where the stones originated, and had no others to compare them to.
So for all their good and grace, the stone was classified by colour. It wasn’t until the late eighteenth Century that crystallographers determined Sapphire and Ruby were the same species.
Now here’s the part when it all falls apart. Sapphires are not just blue.
Corundum is an Aluminium Oxide. There’s a science bit about all that, but essentially it’s Titanium and Iron impurities in the Aluminium Oxide that makes Corundum blue.
If a trace amount of iron is present, the Corundum will exhibit a yellow colour, or even a green colour.
If Vanadium is present it will yield a purple colour. If less than 0.01% of Titanium alone is present the mineral will remain colourless.
For Corundum to be called Ruby, it has to have 1% or greater than 1% of Chromium impurities present.
Up to that point it will vary in shades of Pink and Reddish Pink.
It will still be a Sapphire.
So now knowing what they are, let’s look at where they are.
Kashmir, sitting high in the Himalaya’s yielded the best. The Stones found here had an intense dark silky blue that exhibited the same velvet appearance of the petals of the Cornflower. Hence the desired name ‘Cornflower Blue’.
Burmese Sapphires are also much desired. A beautiful stone, and along with Ceylon Sapphires were also coveted.
Connoisseurs put quite a premium on the stunning pink-orange Padparadscha from Sri Lanka. Blues from here are usually attributed the title Ceylon Sapphire.
They are often lighter and brighter than their Burmese counterparts.
Today the major sources are Madagascar, eastern Africa, Sri Lanka and Australia.
In Part 2 I’ll introduce some phenomena that occur with these stones. Don’t worry it won’t be all technical. I’ll put in some pretty pictures too.
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