Synthetic Sapphires

The very first synthetic gemstone was a ruby made in 1902 by Auguste Victor Louis Verneuil. Until this time, there had been many attempts to synthesise gemstones, but none had been able to duplicate the chemical composition or crystal structure of natural ruby. The so-called Verneuil Technique was the first that actually produced gemstones of high enough quality for use in jewellery. The Verneuil Technique was developed for the synthesis of rubies, but is equally useful for the making of sapphires. The only difference is a change in the starting materials.

The first step of the Verneuil Technique is the preparation of the necessary ingredients. Ruby is a single crystal of corundum (Al₂O₃) with a small amount of chromium impurities (about 2.5%). Verneuil found that by heating Al₂O₃ with Cr₂O₃ , coloured crystals were formed, but these were brown, not the desired ruby red colour. He discovered that this was due to the contamination of the materials by iron. To solve this problem, he used ammonium alum [(NH₄)Al(SO₄)₂.12H₂O] and the analogous chromium alum

[(NH₄)Al(SO₄)₂.12H₂O], which can both be much more easily purified. When heated in a furnace to 1000-1200^oC, the desired Al₂O₃ and Cr₂O₃ are released, whilst the other components are all vaporised at such high temperatures. The resulting powders are then ground together, to form what is known as a 'feed powder'. The preparation of the feed powder remains the same today as it did then.

The second step involves the careful and controlled melting of the feed powder to yield a ruby boule (the term used to describe a synthetic gemstone crystal). To do this, he designed a special piece of apparatus. This has three main parts to it. A mechanism to obtain a smooth flow of the feed powder, a flame for melting it, and a mechanism for lowering the growing boule.

The ruby boule produced, after about 2 hours, is a tapering cylindrical rod about 6mm in diameter and weighing about 15 carats. This boule is under considerable internal strain, so simply tapping it causes it to break into two halves. One boule will provide several faceted rubies, ranging from 1 - 2 carats in weight.

The synthesis of sapphires using this technique is essentially the same, only titanium and iron are used, instead of Cr₂O₃. Whilst the ruby produced by the Verneuil Technique is suitable for use in jewellery, the advent of laser technology in the 1950s, required rubies with higher optical properties. For this reason, flux-growth was developed.

In a platinum crucible, a flux consisting of litharge (mineral of PbO) and a small amount of boron oxide (B₂O₃) is used to dissolve the ruby (or sapphire) ingredients, Al₂O₃ and Cr₂O₃ at 1300^oC. This mixture is then cooled at 2^oC an hour over an eight day period. After this time, the temperature is at 915^oC and the flux is still molten. This can be poured off, and the resultant ruby crystals are freed by dissolving any remaining flux with dilute nitric acid.

This technique produces rubies of vastly superior quality to the Verneuil technique but the need for slow cooling and huge investment for the apparatus (large-scale platinum crucibles are estimated at US$70,000), means that flux-grown rubies and sapphires are much more expensive to make. This means that the majority of synthetic rubies are still made by the Verneuil Technique.

The 1970s yielded a further technique for the synthesis of sapphires. It was discovered that by heating the milky-white mineral geuda in a furnace to a temperature of 1500^oC, a brilliant blue sapphire is formed. Geuda is found in large quantities alongside mined sapphires and has never had a particular use. It is another form of corundum, but lacks the right distribution of impurities to give a pure blue colour. More importantly, however, it contains crystalline needles of titanium dioxide. Heating the geuda releases the titanium from these impurities and titanium ions can enter the corundum lattice. This results in a brilliant blue colour, as described earlier.