Blue sapphire is corundum coloured by an interaction between iron and titanium, specifically an intervalence charge transfer between adjacent Fe and Ti ions, which absorbs strongly in the yellow-red and leaves blue. Very small quantities suffice, on the order of hundredths of a percent, which is why the same mineral that is colourless as pure aluminium oxide can be intensely blue.
Sapphire occurs in every colour; padparadscha, a pinkish orange, is the most valued of the fancy colours and the name is Sinhalese, from the lotus. Star sapphires show asterism, a six-rayed star produced by reflection from oriented needles of rutile within the stone, and the same inclusions in fine dispersion produce the velvety quality of the best Kashmir material.
The important geographical fact is that Sri Lanka (Serendib, Taprobane, Ceylon), has been the classical source for two thousand years and remains a major one. Kashmir sapphires from a deposit worked briefly from the 1880s are the most prized and effectively exhausted. Burma, Madagascar and Montana are other sources.
Heat treatment to improve colour and clarity is near-universal and long-established; beryllium diffusion treatment is a more recent and more contentious process.
In medieval Europe sapphire was the ecclesiastical stone, worn in bishops' rings and read as celestial truth and chastity, and the lapidaries are full of it. That literature requires the caution set out elsewhere in this collection: earlier sources using the word mean lapis lazuli, and the transfer of the name to blue corundum happens gradually through the medieval period, so a thirteenth-century text may mean either.