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I, personally, never spend gems on food. I spend them on equipment to improve my camp. In lower regions it can also make sense to spend gems on playing some of the old stories on sale for 100 gems. They give equipment way better than what you can get in the shop and new events in Egypt and Scandinavia. If you have a strong camp, you can enjoy the game with the energy you get from your equipment in combination with the food you produce. Check out the old events here (scroll down):
https://diggysadventure.fandom.com/wiki/Shop_events
If you want to know more on how to use the old stories to improve your camp we are happy to help. Just let us know, and also what region and level you are in.