“Basically, ruby is a form of aluminium oxide (Al2O3), or corundum, which at nine on the Mohs scale sits just one place lower than the hardest element diamond.”
Sloppy journalism. Diamond is not an element; it is formed from carbon. Likewise aluminum oxide is not an element.
Pure diamond is 100% carbon, which is an element. Diamond is just a rare, crystalline form of the element carbon.
The sentence structure is as awkward as a three-legged goose, but the science is not really wrong.
Diamond is a form of the element carbon. However, the ruby certainly is a mineral as is its first cousin sapphire.