Compound Interest: The chemistry of colour-changing alexandrite

The colour of alexandrite — Click to enlarge

Decades of marketing have placed diamonds on a pedestal above other gemstones. They’re an engagement ring tradition and are also found in lots of other jewellery. However, there’s another, coloured gemstone which is arguably superior. Not content with being one colour, it can display a whole range of hues, depending on the light falling on it. The gemstone in question? Alexandrite.

Alexandrite is a rare form of the chrysoberyl mineral, which has the chemical formula Al2BeO4. Deposits of alexandrite were first discovered in the 1830s, in the Ural Mountains in Russia. Sources attribute its discovery to Nils Gustaf Nordenskjöld, a Finnish mineralogist who initially thought that the gemstone was an emerald. However, he was perplexed by its hardness. Gem hardness is measured on the Moh’s scale, with alexandrite rating at 8.5 to emerald’s 7.5-8.0. Nordenskjöld also noted that the gem changed colours in different lights, appearing red when he examined it by candlelight. He recorded it as a new variety of chrysoberyl, suggesting the name ‘diaphanite’.

Unfortunately for Nordenskjöld, his suggested name didn’t stick. In the gemstone’s Russian homeland, another mineralogist, Count Lev Alekseevich Perovskii (who had sent Nordenskjöld the sample he had evaluated) had designs on naming it. He presented the gemstone to the future Tsar Alexander II on his 16th birthday and named it in his honour.

Alexandrite’s colour is a consequence of impurities present in its chemical structure. Its chemical formula is Al2BeO4; however, in some places where an aluminium ion should sit in the structure, a chromium ion can be found instead. These impurities account for less than 1% of the aluminium sites in the structure – but this is enough to give alexandrite its blue-green hue.

As we’ve already noted, alexandrite’s colour doesn’t remain the same and can vary when its placed in different light sources. In daylight it’s blue-green, but under the artificial light in your home it takes on a purple-red hue. The small amounts of chromium impurities cause this curious effect. Chromium ions absorb visible light strongly in the dark blue and yellow regions of the spectrum. Uniform white light (with equal contribution from all colours) gives alexandrite a purple-grey colour due to this. Other types of light vary in their composition.

Sunlight doesn’t have a uniform contribution from all colours in the spectrum. In fact, there’s slightly more green and blue light than there is red. As more green and blue light remain unabsorbed than red light, alexandrite appears blue-green. In incandescent light, candlelight, or any light which has what we’d term a ‘warm’ temperature, there is a much greater contribution from the red end of the spectrum. There’s also much less blue and green. As chromium ions don’t absorb much red light, this leads to the purple-red colouration of alexandrites in these conditions.

It’s even possible to get one more colour from alexandrite. If a UV lamp is shone on an alexandrite, an intense, glowing red colour is seen (shown below). This also originates from the chromium ion impurities in the structure, but the explanation differs. In this case, it’s fluorescence causing the colouration. The electrons in the chromium ions absorb the UV light, gaining energy and jumping to higher energy levels. As they fall back down to their original energy level, they emit their excess energy as light. This gives the glowing red fluorescence.

If you’re now fully convinced of alexandrite’s superiority to diamonds, there’s some bad news: it’s much rarer and pricier. That said, synthetic versions are available which are cheaper. Some synthetic alexandrites are closer to the naturally-occurring versions than others. Many gemstones offered up as synthetic alexandrites are really corundum (the mineral from which rubies and sapphires are composed) laced with vanadium impurities to give a degree of colour change. These imitations lack the green colours of true alexandrite.

Other synthetic alexandrites mimic the original gemstone’s colour changing abilities more faithfully. There are two methods used to produce synthetic alexandrites: the ‘pulled crystal’ (or Czochralski) method and the ‘flux-melt’ method.

In the ‘pulled crystal’ method, gemstone constituents are melted in a crucible. Then a small seed crystal is lowered into the melt. As the seed crystal is slowly raised from the molten solution, it increases in size as constituents from the solution crystallise onto it. Alexandrites produced using this method have intense colours and are near-flawless. This distinguishes them from naturally-occurring alexandrite, which often contains small imperfections. A derivative version of this method is the ‘floating zone’ method (where the seed crystal is pulled through the solution horizontally instead of vertically).

The ‘flux-melt’ method for growing synthetic alexandrite uses a molten flux of variable composition. The alexandrite constituents are dissolved in this flux. While dissolved, the components can attach to a seed crystal placed within the flux, or crystallise randomly if no seed crystal is present. The process can span up to twelve months, with the slow cooling more adequately mimicking the natural processes that form naturally-occurring alexandrite. Small amounts of flux can be trapped in the alexandrite crystals that form. This makes them harder to distinguish, as they appear similar to the inclusions in real alexandrite.

Whether the alexandrite in question is naturally-occurring or synthetic, it’s unlikely to be taking diamond’s place as the majority’s gemstone of choice any time soon. It definitely makes for much more interesting chemistry conversations, though!