Corundum Formation – How do rubies and sapphires form?

Corundum Formation – How do rubies and sapphires form? 

Corundum Formation – How do rubies and sapphires form?

Did you know that rubies and sapphires are made of the same thing?

Two parts aluminium to three parts oxygen, both gemstones are varieties of what is known as corundum.

Corundum deposits are some of the most widely dispersed of all gemstones.

The most significant deposits can be found in



Sri Lanka (Ceylon)

Myanmar (Burma)






and last but by no means least


Wow! lots of different places spread right across The World.

So, how did this gem end up spread across such disparate areas?

To paraphrase a well known hair advert of many years.

“Here comes the science bit”

The answer lies in plate tectonics.

Corundum Tectonics

Mozambique Belt

Almost all inorganic gemstones are formed in the Earth’s crust.

The few famous rule breakers are



which form deep in the mantle.

But almost every other inorganic gem is formed in the crust.

As a result of the heat and pressure associated with huge tectonic events when continents collide.

These tectonic events bring together diverse rock types.

And with them a diverse range of possible chemical reactions to create a greater variety of mineral formations.

We have as many gemstones as we do due to a happy coincidence.

Life Forms

Life forms began to grow beyond microscopic size around 635-542 million years ago.

As these sea creatures died their remains created a layer of oceanic sediment eventually forming limestone.


At around the same time the Pan-African orogeny (the plate movements creating the supercontinent known as “Gondwana”) brought together most of the continental crust on the surface of the planet at the time.

tectonic plates
Corundum Formation – How do rubies and sapphires form?

The limestone created in the oceanic crust was crushed between these continental crusts.

The former Mozambique Ocean closed up, causing the limestone layer to metamorphose into marble.

This marble came into contact with clay soils during this orogeny.

This was essentially an acid + base reaction, which combined with the heat and pressure of tectonic collision created the perfect conditions for gemstones (and particularly corundum) to form

This gemstone rich area is known as the “Mozambique Belt”.

The Pan-African orogeny produced corundum deposits in





Sri Lanka


Southern India.

These are around five hundred million years old.


Almost all other Ruby and Sapphire deposits were created in the Himalayan orogeny which occurred 50 million years ago, making those corundum stones around a tenth of the age of those from the Mozambique Belt.

This huge tectonic event, when the Indian plate separated from Gondwana and slammed into Central Asia at (by geological terms) a phenomenal speed created the mountain ranges of the Himalayas and the Earth’s highest peak, Everest.

Fossilised sea creatures have been found in the limestone at the summit of Everest, testament to the truly extraordinary forces involved.

The Himalayan orogeny produced pockets of corundum deposits stretching across




Northern India


Myanmar (Burma)




In other areas, notably, Thailand, corundum was formed through volcanic action.

Corundum Chemistry

The chemists, among you, maybe wondering how corundum, which is a crystalline form of aluminium oxide (Al2O3), forms from the interaction of marble and clay, let alone how corundum of a variety of colours can form.




It is thought that some clay on the periphery of continental crusts metamorphosed into shale.

During this process, hydrothermal solutions containing the necessary aluminium as well as such trace elements of chromium or vanadium were deposited in the shale.

This shale became layered through the marble and crystals of corundum grew on the boundaries between these rock types.

To complicate things further in some corundum mines the gem spinel is also mined.

The Luc Yen and Magok mines in the Himalayan Belt and the areas around Mahenge in the Mozambique Belt produce both corundum and spinel.

Which is probably why the spinel in the Imperial State Crown of the British Crown Jewels was for centuries thought to be a Ruby and is still referred to as the Black Prince’s Ruby.

More science bits.


The reason for this is that there are actually two different varieties of “marble”.

The most straightforward is simply calcite (CaCo3) which combined with the aluminium deposited in shale can create corundum of varying colours.

The “other” marble is actually dolomite (CaMg(Co3)2).

As you can see this is a more complex rock containing magnesium (Mg).

When dolomite and shale come together under the high temperatures and pressures associated with these huge geological events, the aluminium in the shale and the magnesium in the dolomite join together to form spinel (MgAl2O4).

It is only when all of the magnesium is used up that corundum can form in these locations.

This perplexingly means that in dolomite-marble gem rich area spinel is more abundant than corundum.

However, since there are more calcite-marble gem rich areas in total, spinel is more rare than corundum overall and can command a much higher price per carat.

This does not mean that corundum isn’t rare, however, fine quality rubies and sapphires are some of the more sought after gemstones on the planet, fetching astonishing prices of their own.

They are, also, far more rarer than Diamonds.

When it all boils down to it , although Mother Nature is wonderful and I find all the science bits really interesting at the end of the day most of us girls, myself included, just want a big sparkler.!!!!!


Source: Gemporia Magazine




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