How do you activate gems in Sonic the Hedgehog

Press up or down to pick and press R1 to use it.

P.S. Try to do all the town missions because if you miss them, you can't come back to them and theres no other way to get money. And you want money because otherwise you can't buy gems, and you will want gems

P.P.S. If you use the purple gem, you can glitch the game. While you are using the gem, you can jump again and again. And it is like flying. Don't buy the green one. It is useless.