In QI S17E06 Quests part 1 Sandy claims that The Black Prince's Ruby is the worlds largest uncut ruby - but it is in fact not a ruby. It is a spinel and the name is just a result of people not being able to differentiate between them until they could be tested chemically.
The spinel is a compound of magnesia, iron, oxygen, and chromium, while a ruby is a type of aluminium oxide.
In the top of The Black Prince's Ruby there is however a very small true ruby embedded to fill a hole drilled when it was worn as a pendant.
I remember this fact from a documentary about the British Crown Jewels that was the first thing I ever recorded on a VHS tape... Probably in the first half of the nineties when I was in my teens.