What alternative resource could I use instead of crystals which are consumable/not reusable and can't be just found and used?

My idea for my next campaign is that The Empire uses crystals to heal everyone each day and soon there won't be any crystals left on the continent, so the pc-s should go on an adventure and find some. I'm quite happy with my idea besides the fact that it's some crystal... again... for the 100th time.

Are there some more creative ways to look for something, which

• does not replenish
• can't be just collected and used immediately (it has to be mined or the thing is just one ingredient of several or whatever)
• could be found by using a magical artifact which kinda acts like a compass but instead of the north it points to the closest supply of this thing?

What is your "special resource to fuel an empire"? Oil seems also a bit boring.

EDIT: the fact that these crystals can be used on the level to support thousands or even millions and still can not be reproduced is the most important part.