Gems are the premium in-game currency used to advance players' villages. When players first download the game they begin with 500 Gems (250 of which you are forced to use during the tutorial, and another 8 if you rush the buildings and troops when prompted). Additional Gems can be earned in-game by completing Achievements or clearing Obstacles, or purchased with real money. Gems can be used in several ways:
Gems are available to purchase in the Treasure menu of the Shop. For more information on prices in multiple currencies, please visit In-App Purchases.
Gems can be earned in the game by completing Achievements. When you complete challenges in the Achievements menu, for example, 'clear 50 obstacles', players are rewarded with Gems. When players gain 3200 trophies, they will be awarded 2000 Gems (3715 if you count all the Gems awarded by trophies). This is the most amount of gems players can earn from achievements, use them wisely.
Another way to earn Gems is to clear obstacles, which are a never-ending (albeit slow) source of Gems. Obstacles will give Experience and anywhere between 1 and 6 Gems whenever they are cleared, although sometimes they may not give any gems at all. There is also a special obstacle known as a Gem Box, which always yields 25 gems when cleared, but they are much rarer than normal obstacles.
A further (although limited availability) way to earn Gems is to be one in the Top 20 players of the Top 3 Clans. At the end of each two-week period, the Top 3 Clans earn 50,000 Gems for 1st, 30,000 Gems for 2nd and 15,000 Gems for 3rd; these totals are divided equally amongst the Top 20 players of the respective Clans.
Gems can be mined in the Gem Mine, which is in the Builder Base, though the process is slow.
Clash of Clans is free to play, but contains a premium currency (in this case Gems) available for purchase that gives players in-game advantages. These type of games are called 'Freemium' games.
As players buy larger amounts of Gems, the cost per Gem decreases.
Many (if not most) high-level players buy large quantities of Gems (these players are often called "Gemmers"); Jorge Yao, who was one of the top players in the world, has stated in interviews that he had spent over $2,500 buying Gems.
It takes much more gems to heal a hero or to request Clan Castle troops than to finish upgrades or troop training: for example, it takes 7 gems to finish an upgrade or troop training with 20 minutes remaining, but it takes 42 gems to request Clan Castle troops again immediately after requesting and 23 gems to heal a hero with 20 minutes remaining.
Between November 24, 2014 and December 8, 2014, iOS users were able to purchase a pack of (RED) gems for $4.99 to contribute to AIDS research. When they did they received a red shield on their Town Hall.