Gemstones are beautiful; it is one of the things that defines them as gemstones, but some have ‘party tricks’ that make them extra special. One such group is colour-change gems; stones that appear a different colour depending on what lighting conditions you view them in. We currently have a couple of gems in stock that do this, so it seems like a perfect time to explore how they perform this little bit of magic…
The most famous colour-change gem is Alexandrite, a rare variety of the mineral chrysoberyl that wowed the miners who first discovered it in the nineteenth century when they noticed that the green gem they had found suddenly appeared red in the evening candlelight.
Fine quality Alexandrite, especially material that actually displays a full and clear change in colour, is incredibly rare and valuable. Stones that have a more subtle shift in hue can still have prices in the thousands if the stone is attractive; the stone above has a greenish-blue to purplish-blue change, weighs 0.83ct and would cost £4000-£5000 in a ring. Even the name is desirable, and we see tiny, brownish chrysoberyl stones with a tiny hint of shift in the shade of brown, labelled as Alexandrite to make them more attractive to the purchaser.
While no other stone offers quite the ‘wow’ factor of the finest Alexandrite, there are other gems that also have the ability to change in colour and these can be much easier to access for the ordinary consumer. Out first magician is a sapphire which appears green in daylight and blue under incandescent lighting with a combination teal hue under mixed lighting conditions. Colour is very difficult to photograph, but you can see the definite change in the images above – in person the green is brighter than pictured. In the box on the left the ring is under fluorescent tube lighting while on the right it is under a daylight lamp.
We have previously stocked a sapphire that changed between purple and blue, and a stone dealer recently showed us some sapphires that had various combinations including one that had a fabulous raspberry-pink colour that became violet under our daylight lamps (the stone pictured above which photographs slightly differently!).
Another ring we currently have in stock is a garnet which has a change from a light golden brown to a pretty pastel pink. Under daylight the pink is rosy and very beautiful while in mixed lighting the warm golden colour has a lovely rich feel.
So, why do these gems appear to change colour under different light sources? This is where we need to refresh a bit of science from school!
What we perceive as ‘white’ light is actually made up of many different wavelengths that represent all the different colours of the spectrum. You may remember using a prism to split white light into a rainbow – this perfectly shows you how light is made up of all the different colours.
The reason we see colour in objects is because they either absorb or reflect different wavelengths of light. An object that absorbs all wavelengths is black, one that reflects all wavelengths is colourless or white. For other colours it may be very simple – an object absorbs all the other wavelengths except red and so we observe it as red. Mostly though, the situation is a little more complex and the colour we see is created from a combination of all the reflected wavelengths. Very subtle differences in the exact wavelengths absorbed can completely change the colour. Rubies (red) and emeralds (green) absorb light very similarly but appear completely different colours! Red and green are opposite each other on the colour wheel and this means they balance each other. In this case, rubies appear red because they absorb more green light.
While daylight (sunlight) contains a balance of all the different wavelengths, many artificial light sources are actually composed of different ratios. Candlelight is a little stronger in wavelengths towards the red end of the spectrum. Fluorescent lights often contain wavelengths concentrated in just one part of the spectrum.
Many gems shift colour slightly when you look at them under daylight and then candlelight, the increase in red wavelengths just altering the hue slightly, but for some gems the change is much more obvious, giving impressive colour-changes that can be exciting to observe. The reason Alexandrite can change so dramatically is that it has similar absorption to ruby and emerald – but sits just in the right place to give a green colour under the balanced white light of daylight. Under the more strongly red candlelight there is just enough change in that balance to completely alter the colour. Essentially it boosts the amount of red light and tips the balance in favour of that colour.
You can view our colour-change sapphire and garnet rings in our showroom, where you can talk to our jewellers and gemmologists about these amazing stones. If you have an enquiry for another colour-change gemstone we may also be able to help. Just visit us or get in touch to ask any questions, we’d be delighted to help.