Sapphires are one of the main currentices in Million Lords, they can be either earned by unlocking achievements, doing daily/weekly quests, watching ads, and harvesting on the map or bought from the store.
You can spend sapphires in the store for many things such as chests, advisors, city skins, gems or bundles.
You can also use them to improve your gameplay with things such as using special multi-scout, troop regroups, reseting your skill tree or speed boosting your troops.