How do you use gems in Sonic the Hedgehog?

Answer 1

Press left or right on the D-pad and watch the energy gauge. A colored icon should appear and Sonic's shoes will change color. There is sometimes a delay, so just press left or right repetitively until it works. You can use the ability with R2 (Or for the Xbox, the button in it's place.)

Answer 2

The original Sonic the Hedgehog, You CAN'T. If you collect all of them and complete the game you get a slightly different ending, but that's it. A use (super sonic) wasn't thought up for them until sonic 2.