One of the most remarkable things about the formation of rubies is that geologists are not sure how it happens. Without question, however, the very existence of rubies is something of a minor geological miracle.
Rubies are a red variety of a transparent corundum. After diamond, Corundum is the hardest mineral that exists. It is made up of densely packed aluminum and oxygen atoms, which are normally colourless. Corundum is found in two varieties, common corundum, which is impure, coarse, opaque and granular (known as EMERY) and transparent corundum, which includes the RUBY and SAPPHIRE (both identical except in colour). When other atoms are substituted for a few of the aluminium ones in the colourless corundum, bright hues emerge. Small amounts of chromium impart the deep red color of ruby; traces of titanium and iron produce the stunning blue of sapphire. However, if there are large amounts of silica and/or iron present then you will not find rubies or sapphires in the corundum.
Consider the fact that it is extremely rare to find corundum in the first place. Now consider the fact that silica and iron are two of the most abundant elements in the Earth’s crust and chromium is exceedingly rare. How did rubies manage to avoid the commonplace silica and iron but miraculously find some chromium? Sapphires do need some iron but the fluorescence of rubies depends on very low quantities. This unique set of circumstances is what makes rubies that much rarer.
The commonly held belief amongst geologists is that rubies are formed by tectonic plates smashing together – as did the India and Asia plates when the Himalayan mountains were formed around 50 million years ago – forcing limestone deposits deep into the earth where intense heat and pressure metamorphosed the limestone into sparkly marble. At the same time, molten granite bubbled up into the marble and removed the silica but left behind the aluminium through a process called metasomatism.
The problem is, and the part that is confusing geologists, is that although the ruby-hosting marble extends over large areas of the Himalayas, the rubies themselves only appear in sporadic, localised patches. There must be some other missing piece of the puzzle that determines why and where rubies will form in this marble. This confusion was compounded by the discovery of some of the world’s most impressive rubies in the Mogok mine in Myanmar (formerly Burma). Whilst the rubies were hosted in marble, they were found alongside topaz and moonstone, minerals that are igneous rather than metamorphic in origin. This has confused geologists further and it may be many years before they find a final definitive explanation for how these beautiful gemstones are formed.