How do you get a sapphire in RuneScape

Mine ore like coal or mithril, and it will randomly give random gems including sapphires. If you have high enough crafting level, you can cut it with a chisel.
You can also buy from the Grand Exchange and rarely from Gem Stalls.
It can also be dropped from certain monsters.

If your a member you can mine them from Shilo Village, after completing the
'Shilo Village' Quest.