Gems such as Ruby, Sapphire, and Diamond are some of the more rare and needed resources in Hyrule Warriors: Age of Calamity. Most of the resources are needed to either level up a certain character, unlock a special item, or progress through quests and could take time to find and grind if you go about killing enemies and mining rocks on the battlefield to acquire them.
Thankfully, Koei Tecmo were aware of that when making the game and came up with a solution for players looking to get the Gems far easier. After progressing through a portion of the main story, you will unlock a Store over near Gerudo Village that will allow you to buy any gem you want for a massive rupee price.
The store is unlockable like most stores on the map through items but unlike the rotating stables, this one will always sell Gems. These Gems, once bought, will be restocked after a few in-game days, allowing you to buy as many as you need to continue your quest.