There is no official generator capable of adding gems to a Clash of Clans account. Websites promising unlimited amounts do not communicate with the game servers and may be trying to collect personal information.
Legitimate sources of free gems
Remove obstacles from your village, complete achievements, take part in Clan Games and check event rewards. The Gem Mine in the Builder Base also produces gems over time.
The best early-game use
Save your gems for Builder Huts first. An extra builder speeds up every stage of your long-term progress, while finishing one upgrade instantly saves time only once.
Protect your account
Never share a Supercell ID code, password or token received in the game. A legitimate analysis tool needs only your public player tag.