Opinion: What science can tell us about the 'world's largest sapphire'

January 7, 2016 by Simon Redfern, University Of Cambridge, The Conversation
Opinion: What science can tell us about the ‘world’s largest sapphire’
How did the Star of Adam grow so big? Credit: BBC

The "Star of Adam", recently found in a mine in Sri Lanka, is believed to be the biggest sapphire ever discovered. It weighs in at over 1,404 carats, that's around 280g or just under ten ounces. But what do we know about the formation of this remarkable gemstone – and how could it grow so huge?

Sapphire is a bright blue gem mineral and a form of corundum (aluminium oxide), the hard gritty stuff used as an abrasive in emery paper. It is incredibly hard – a fact important in understanding its occurrence in places like the Sri Lankan mines.

Sapphire is a type of "dirty" corundum. If you add just a trace of iron and titanium to the mixture of aluminium and oxygen from which the corundum is growing, it forms as . (If you add chromium to the corundum as it grows then you will get a ruby – Sri Lanka is also famous for its rubies).

The Star of Adam sapphire is an example of a "star sapphire". When you look at it it appears to have a six-pointed star inside, which shines out from the gem and is due to reflections of light from tiny whisker-like crystals of rutile (a titanium-dioxide mineral) that were trapped within the sapphire crystal as it grew.

Ancient river sediments

The stone was found in theRatnapura mines in the south of the country, about 100km south-east of the capital, Colombo. Ratnapura is Singhalese for "gem town" and Sri Lanka has been known for its gem deposits for more than 2,000 years. It seems likely that Sinbad's "Valley of Gems" in the Tales of the Arabian Nights is a reference to the Ratnapura area. In 1292, Marco Polo wrote: "The Island of Ceylon is, for its size, the finest island in the world, and from its streams come rubies, sapphires, topaz, amethyst and garnet."

Ratnapura gem mine. Credit: hassage/Flickr, CC BY-SA

The gems of Ratnapura are found in ancient river sediments – old river beds that are now covered with more layers of mud and sand in an area that is largely given over to paddy fields. The hard gem minerals, sapphires, rubies, spinels and garnets, were long ago weathered and eroded from the nearby highlands. Because of their hardness they survived as large pebbles and crystals, eroded out of the rocks where they first formed, and transported down the rivers which acted like a natural panning system. River-borne (alluvial) gold and diamonds are often sorted and concentrated in river sands by similar processes, elsewhere on the globe.

On their journey along the river the softer rocks from the highlands would have been worn down into mud and fine sand, but the harder minerals survive better, and retain their size and often their shape. The average annual rainfall for the island is more than 2,000mm, and the tropical weather means that the erosion and weathering of the highland mountains is even more accelerated.

Ratnapura is in the "wet zone" of the island. Its gem-bearing gravels have yielded a number of historic gemstones, possibly including a 400-carat red spinel given to Catherine the Great of Russia, and a giant oval-cut spinel, known as the "Black Prince Ruby" (it was mistakenly identified as a ruby), which features in the British Queen's imperial state crown.

Opinion: What science can tell us about the ‘world’s largest sapphire’
The Black Prince’s Ruby, the irregular-shaped red stone at the front, is actually a spinel.

The Star of Adam sapphire would originally have been created within rocks and granites of the Sri Lankan highlands. The granites, which form when molten magma cools and becomes solid, have been dated as almost two billion years old, and were subsequently squeezed and re-worked in a massive mountain-building episode due to tectonic churning of the Earth's crust that happened more than 500m years ago.

Temperatures and pressures deep within the roots of these mountains would have reached more than 900˚C and over 9,000 atmospheres pressure during this event. The sapphire could have formed either within the granite, as part of a rock type called a pegmatite, or within the younger rock created by pressurisation and heating.

In either case the temperatures and pressures would have changed only very slowly over millions and millions of years, and this is how the crystal was able to grow so big. Once formed, the mountains that it sat within would have been eroded and uplifted, and so it was brought to the surface, picked out of the rock by the forces of rain and weathering, and transported down river to the gem sands of Ratnapura. Today it sits in the hands of a private owner.

Simon Redfern, Professor in Earth Sciences, University of Cambridge

This article was originally published on The Conversation. Read the original article.

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TheGhostofOtto1923
3.7 / 5 (6) Jan 07, 2016
I wonder what it looked like uncut?
ogg_ogg
3 / 5 (4) Jan 08, 2016
Been away for a while. Read this trite NON-information and wondered WTF? Then I saw where it was from...ahh, "The Conversation", I'd forgotten that Phys.org has some sort of relationship with this provider of garbage. Well, it could be this, or it could be that. But let's not mention why it couldn't be a billion other things, and lets simply explain that it could grow so big because it was hot for millions of years ... unlike other places, I guess they are trying to (erroneously) imply. Too bad Phys.org can't be honest and label this filler not as "opinion" but as what it is: vacuous.
gkam
1 / 5 (6) Jan 08, 2016
What makes otto think it is crystalline? Ever had one? They are soft, not rigid.
Uncle Ira
4.3 / 5 (6) Jan 08, 2016
What makes otto think it is crystalline? Ever had one? They are soft, not rigid.


Why don't you ask Otto-Skippy instead of the rest of us?

Well anyhoo, I am not very busy right now so maybe I can help Cher.

Hmm, that is tough one. Well maybe he actually read the article instead of just making up some silly thing like glam-Skippy did, AGAIN.

The Star of Adam sapphire is an example of a "star sapphire". When you look at it it appears to have a six-pointed star inside, which shines out from the gem and is due to reflections of light from tiny whisker-like crystals of rutile (a titanium-dioxide mineral) that were trapped within the SAPPHIRE CRYSTAL AS IT GREW.


Did you study in the moron school? I bet you got the Master Diploma in that.
gkam
1.5 / 5 (8) Jan 08, 2016
It is fun to bait Ira now. With no life of his own, he follows me around taking shots because I beat him at his gotcha game.

I can get him to look up anything.

Uncle Ira
5 / 5 (6) Jan 08, 2016
It is fun to bait Ira now.


Now there you go Cher. That's what I been trying to explain to you all along. Relax and enjoy life, all this stuffs is is some Skippys on the interweb fooling around.

With no life of his own, he follows me around taking shots because I beat him at his gotcha game.


What is that supposed to mean Cher? Saying you beat anybody is whistling by the bone yard. Just like this comment page we are on now. Maybe you think you won again, but all you done is show how stupid you really are. You are not very good at the insulting thing non. You should realize that you just are not very good at this.

I can get him to look up anything.


Well that is good theory, except for one thing. You make the comment to Otto-Skippy, not Ira-Skippy. Except for another thing. Nobody had to "look up anything", all they had to do was read the article before they make the comment. Like you did not do, AGAIN.

This is you beating me again.
Vietvet
4.3 / 5 (6) Jan 08, 2016
I wonder what it looked like uncut?


The owner said he bought it cut and polished, it's unlikely we'll ever know what the uncut stone looked like.

It's larger than the photo suggests.
http://www.thegua...nka-300m
TheGhostofOtto1923
4 / 5 (4) Jan 09, 2016
I wonder what it looked like uncut?


The owner said he bought it cut and polished, it's unlikely we'll ever know what the uncut stone looked like.
When theyre uncut you get to see their structure, variations in color, inclusions and flaws, and possibly the matrix. Much more interesting.
https://www.pinte...8218602/
What is that supposed to mean Cher? Saying you beat anybody is whistling by the bone yard. Just like this comment page we are on now. Maybe you think you won again, but all you done is show how stupid you really are. You are not very good at the insulting thing non. You should realize that you just are not very good at this
Like george says hes baiting you.

Psychopaths are master-baiters.

George gets his kicks from manipulating, especially himself. Its all hes got left you know.

Read more at: http://phys.org/n...html#jCp
AGreatWhopper
3 / 5 (2) Jan 12, 2016
I wonder what it looked like uncut?


That's what she said.

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