The causes of colour in beryl have been a research topic for decades. For some varieties, such as emerald (green, coloured by Cr 3+ and/or V 3+ ), the main cause of colour is substitutions by metal atoms within the framework. However, the causes for the yellow and blue colours in heliodor, golden beryl and aquamarine are still debated. It is generally agreed that Fe ions are responsible for the colour, but there are differing...