Gems are a type of in-game currency. There are three kinds of gems: topaz, emerald, and ruby. There are several ways to get them:
1. Send fully-loaded airplanes. You're guaranteed to receive one gem as a reward.
2. Explore the mine. Gems can be found in treasure chests or discovered in earth tiles.
3. Open treasure chests in your town and your friends' towns.
4. Play at the House of Luck to win gems as prizes.
5. Take part in weekly regattas — you may receive gems as a reward.
6. Buy gems any time using Township Cash. You just need to open the Store and click the arrow sign next to the gem icon at the bottom. Then you’ll see the options available for purchase.

You need gems to buy lab boosters, unique decorations, and animal cards at the Zoo Shop.