A variety of corundum—aluminium oxide. Pure corundum is colourless; trace titanium and iron turn it blue. Ruby is its sibling: chromium produces red, titanium and iron produce blue. The same mineral, different colour.

The Kashmir deposit, discovered in 1881, was exhausted within a few years, yet those sapphires still define the benchmark for quality. A stone that can no longer be mined, still setting the standard. Current sources include Sri Lanka, Myanmar, and Madagascar.

Mohs hardness 9—second only to diamond. That hardness has made it useful in watch bearings and industrial applications. A mineral chosen not only for beauty, but for physical performance.