Almost all obstacles in Clash of Clans have some gems in them; you can get anywhere from one to 25 gems(Gem Box) from an obstacles. As long as you keep cleaning obstacles, more keep coming in their place.
Never, ever believe in those "Free Online Gems Generators" - they are SPAM. Supercell makes money selling gems; they won't let anyone cheat their game. Even if you get free gems by cheating online, you might be banned from Clash of Clans, or even have to start all over again.
Edit(Jan-2015): Some Clash of Clans Gem Generators do work. The person offering the gems, buys the gems for us on our account. However, be aware that most of them are still spam. Some honest generators, really buy us some gems from the Clash of Clans store legally and give it to us.
Free Clash Of Clans Gems $500 Gift Card Hacking Video Tutorial :