Gems (these green crystals) are used for a lot of stuff in-game. But remember a few things! :
1) You don't need to buy them with real money!
Just wait to see a new game, which can be DOWNLOADED from Google Play. This action will
give you up to 4 gems, which can get useful sometimes.
2) You have to be careful, when a new ad pops up on your screen! Sometimes, you can
accidentally press "Add" or "Buy Gems" and if you're not disabled "in-app purchases" you can say goodbye
to your precious money! Be careful guys!
And the other tip is - You are not supposed to play with gems, they are not nessecery at all.
You can spend them on cooking food (which is a BIG mistake I had made a lot of times, before I became
a good Restaurant Story player) and this will finnish the process immediately.
You can spend them on decorations, chairs, tables, counters, floor tiles, chests and whatever man! But it doesn't worth it!
1st, because when you buy something with gems, it's VERY VERY cheap, when you sell it later and
2nd, because most things can be bought with coins (with gems too, but they are too expensive though)
Gems can be spent on stoves, ovens, grills (e.g. "Sonic Grill" ; "Forever Stove" etc.) BUT ,once again it doesn't worth it! But after all, it's your choice and taste! Good luck with Restaurant Story.