Gems are the currency that you use to advance in the Clash of Clans game. Every player will start out with 500 gems. Half of the gems will be used up in the tutorial which every player must go through. Whenever it prompts you to rush a building or troop training, you have to spend 8 gems. When you have accumulated some gems, you can use them to purchase some stuff in the game.
You can spend your gems on buying builder huts. Every builder hut comes with one builder. Builder can be used to build and upgrade structures. The more builders you have, the lesser time it takes to upgrade your base. Having a lot of builders allows you to reach town hall faster. There are 5 builders and each builder requires a different amount of gems to obtain.
You start the game with a free first builder. The second builder will cost you 250 gems. The third builder costs 500 gems. The fourth builder costs 1000 gems and the fifth builder costs 2000 gems. If you spend your gems wisely, you should be able to purchase the 4th builder by the time you are at level 6.
After acquiring all 5 builders, you can spend the gems on boosting your resources. For example, you can use 10 gems to boost a barrack, or spell factory for 2 hours. This can increase its productivity to 4 times faster and save you 6 hours. With 11 gems, you can give your gold mine or elixir collector a 24 hour boost to 84,000.
You can get 2,400 Dark Elixir from boosting the Elixir Drill for 24 hours. It takes 34 gems to boost the Elixir Collector for 24 hours. As a word of caution, you should avoid boosting too much as it can cause your builders to experience lootsplosion. You can use gems to buy some time. It will cost you 266 gems on the first day and 123 gems for each subsequent day. Other things that you can purchase with gems are the pirate flag, mighty statue, shields, gold, elixir, and dark elixir.
It takes a few days for a building to be constructed. There is a turnaround and it is to use gems to instantly finish the construction of the building. Just like building construction, it can take days for an upgrade to finish. If you cannot wait, you can use gems to instantly finish the upgrades. If you have a lot of gems, you can upgrade a Townhall 10. You can also use the gems to upgrade your laboratory. Gems can upgrade heroes. After you upgrade the heroes, you must wait for 1 week to use the hero again. You can gem the wait time of your hero if you find the need to use the hero in the war.
Clash of Clans allows you to earn gems in the game or purchase them. Some of the free methods to earn gems are completing the challenges in Achievement, clear obstacles, and completing events. It is important to save the gems and only use them when necessary. You should never have to purchase gems with real money if you cannot afford it. Clash of Clans is only a game and you should not have to face with a financial problem just to have some fun.