Don't worry, if you don't want to open them with Magic Gems, you can sell them to get some Coins!
Tap a chest, then close the popup. The chest should now be selected. Then use the "Sell" button in the info bar on the bottom of the screen, here:
If you don't see this bar at the bottom of the screen, you may need to expand it with the triangle-shaped button on the bottom-left: