I've no idea if this is well-known or not. Was news to me.
Rubies and Sapphires are in essence the exact same thing.
They're both
Corundum, a crystal form of Aluminium Oxide (Al
2O
3).
The crystals get their respective colours based on the percentages of and types of impurities that are found within the crystal.
Rubies usually have an impurity of Chromium found in them, giving the transparent crystal a Red colouring.
Sapphires, while typically associated as being Blue, can be many colours.
You can get Sapphires that are Blue, Yellow, Green, Purple, Pink, Orange and other varying colours that depend on the concentrations of impurities present.
While Sapphires can be Green, don't get them confused with Emeralds, which are infact their own unique type of Gem (Be
3Al
2(SiO
3)
6)