Conundrum

Corundum is an aluminium oxide. Maybe you haven't heard the name before, but you know the stone. When a corundum is red, it's called a ruby. When it's any other colour, it's a sapphire. So rubies and sapphires are pretty much the same thing. It's chromium that gives the red colour, and in sapphires the colour will similarly be influenced by trace elements.

Lots of stones are called semi-precious. Semi-precious stones are ten a penny. For precious, you're limited to only four stones, and oddly, two of these are ruby and sapphire. The other two are emerald, which is a beryl variety, and diamond. Other beryls include aquamarine and heliodor. Why these two don't make the grade I have no idea. Diamond is carbon; the only other example of pure carbon is graphite - pencil lead - and I know why that's not considered precious. These intricacies in nomenclature are at least partially down to the differences between the geological world and the gem business. Probably if it were down to geologists it'd just be red corundum and blue corundum. Green beryl and yellow beryl. Hard carbon and slippery carbon.

Once had quite a long conversation with a customer who had come in wanting a piece of beryl. I said I had either aquamarine or emerald, but he was adamant he wanted just beryl. I think eventually he bought something else.