September — Sapphire

Second-hardest thing on earth, and it decided to be the colour of the sky.

Sapphire is corundum — aluminium oxide, 9 on the Mohs scale, beaten only by diamond. The blue comes from traces of iron and titanium sitting where aluminium should be. Remove them and the stone is colourless.

It has been the September stone since the modern list was set, and a stone of judgement and clear seeing for a great deal longer than that.