What is the chemical structure of sapphire?

Answer 1

Sapphire is a type of Corundum. Corundum is a lattice of Aluminium Oxide, Al2O3

It is impurities within the lattice that give corundum a different colour, upon which it is known under different names. Trace amounts of Iron and Titanium within the corundum give the traditional blue colour, but are not part of the chemical structure as such, imagine them to be like marbles embedded in concrete, at a molecular level.