Many users of Cooking Fever find it hard to earn gems and I admit I do want easier methods for collecting gems myself. But that just means you have to make the most economical use of your gems and coins! So here are some ways to gather enough gems while using them for purchases and upgrades-

1. You get about 4 or more gems after every level of XP so use only one or two for your game work.
2. If you want to get your XP level up sooner, do levels you have already completed (and I don't mean the first 5 levels of the game!!)
3. Finally, if you click the '+' icon with the gems and coins' icon, one label will read 'Free'. It is quite easy to earn about 10 gems through that. :)

Hope you find it all helpful. Cheers!