What are the pink gems for in the upper part of my screen near my "level" count?

In the game, if you click on settings and then get help you'll see that the gems can be used for boosters during game play! I think the reason no one knows this is because they provide an ample amount of free boosters. But that is what they're for!!!

When I try to purchase a booster it only gives me the option to pay with coins.

I think you can use gems to buy boosters once you’re out of them. I haven’t been out of shuffle or hint boosters yet but I’ve been trying not to use them. I’m going to be more free with use and try buying more with gems when I run out.