The Science of Gemstones

A crystal of pure corundum would be colourless. But even the tiniest dash of an impurity imparts a vivid colour. If the crystal picks up traces of chromium, it glows with the deep red of a ruby. Any other ingredient, tinting the gem with another colour, gives us a sapphire. Though we think of sapphires as blue, they can range in colour from yellow to green and purple. The rare Padparadsha (“lotus flower”) type is a lovely orange sapphire.

Not a ruby but a rare orange sapphire

These wonderful gems are found in only a few locations on Earth, largely in a band that stretches around the Indian Ocean from Madagascar and Kashmir to Burma and Thailand. Australia and Montana also weigh in with sapphire deposits.

Aluminium and oxygen are among the commonest elements in our planet, so it may seem odd that rubies and sapphires are so rare. But geology is never simple. Silicon is also a major element in the Earth. If there are silicon atoms around, the aluminium and oxygen atoms will pull them into the molecule as well, to make minerals (like feldspars) that are more complicated, but less beautiful and valuable.

So the search for rubies and sapphires becomes the search for crucibles in the Earth where silicon is missing. The raw material is often the debris from granite outcrops that have been weathered away. The process washes out compounds of silicon compounds, leaving an insoluble white clay - a soft compound of aluminium oxide and water. It’s called bauxite, after a huge deposit found at Les Baux in France.

In Australia and Thailand, ancient deposits of bauxite have been caught up in the upheaval of plate tectonics. The bauxite has descended over fifty miles into the Earth. Here, the rock is at squeezed at thousands of atmospheres of pressure, in temperatures of hundreds of degrees. Water is driven out of the bauxite, and the aluminium and oxygen atoms are crushed together to form corundum. Tiny traces of other atoms create the whole panoply of sapphire colours, from blue through to yellow.

But the world’s biggest hoards of ruby and sapphire come from more complex geochemical cauldrons.

In the jungles of Burma lies the deep valley of Mogok. From time immemorial, local people have known it’s the source of magnificent rubies. Yet there was a problem. The valley is full of lethal snakes. The Burmese harvested the rubies by throwing down hunks of meat. Rubies became embedded in the meat; birds of prey carried off the meat and devoured it; and the local people gathered the rubies from the birds’ droppings!