I wondered this too, when I was starting out. As far as I was aware, purple meant amethyst. Didn’t it? 

Well, as ever the answer is that it’s not quite as straightforward as that. Certainly some gems do have characteristic colours, and that can be very helpful in determining their identity, but you can easily be caught out. I was recently asked to make a commission with three lovely purple gemstones, which were bought by the client’s father for her mother around 25 years previously while he was serving in the merchant navy. We didn’t really know where they were from, or what he had paid for them. They looked purple when I was shown them, and my first thought was amethyst. But there was something nagging at me.

When I got them home into my studio I took my gemmology torch out to have a closer look. When I shone it on them to look at the stones more closely they looked deep pinky red, not purple. How could this be?

Well, when I first saw them in daylight, they did look purple, but a slightly different shade to what I would have expected from amethyst. The red appearance with my gem light fitted with my suspicion of a colour change gemstone. These interesting stones appear to change colour in different types of light, but actually they stay the same, it is our brains that perceive them differently. Daylight is rich in blue light, and the tungsten light in my gem torch (and many old fashioned lightbulbs) is rich in red light. The difference in the light reflected by the stones under these two conditions is sufficient for our brains to be tricked into interpreting that as a difference in their colour. Fascinating, isn’t it?! 

These gems turned out to be colour change synthetic sapphires, and went on to become three lovely necklaces for three different family members.