Ruby
Ruby is aluminium oxide, with trace amounts of chromium that give it a distinctive pink or red colour — Al2O3:Cr. With more chromium, it's red. With less, pink. With even less, it's purple or blue, and called sapphire.
It's especially hard, scoring an impressive 9.0 on the Mohs scale, making it the ideal material for a mechanical watch's jewels. (At a standard-setting 10, diamond's even better, but prohibitively expensive.)
Ruby also has various properties that make it an ideal gain medium in lasers and masers, and indeed it was used in the first ever laser.
Nature doesn't happen to make very much ruby. Thankfully, synthetic ruby can be grown in a factory out of regular aluminium oxide and chromium for a fraction of the cost of mining natural ruby.
Mechanical watch materials: Ruby | Synthetic ruby
