Diamonds are a very important currency in AFK Arena. There are many ways to obtain diamonds, both for free and for money.
One of the most important way of spending diamonds is to summon heroes in the Noble Tavern whether you are spending one summon for 300 diamonds or ten summons for 2700 diamonds.
Diamonds can be also used to claim additional AFK chest rewards (50 diamonds, increases with consequent uses).
You can also spend diamonds to buy in general store and refresh all the stores (prices vary from store to store as well as it's increasing with consequent refreshes).
You can reset a hero for 20 diamonds, refresh your bounties for 50 diamonds or remove cooldown in Resonating Crystal for 100 diamonds.
Good source of diamonds available from early on and deep into your progress are Campaign quests.
You can get diamonds for passing campaign milestones (e.g. xx-4, xx-8 etc.), level milestones (e.g. level 100, 120 etc.), reaching player levels and many more.
You will also get 10 diamonds for every player level you achieve.
A decent way to earn some diamonds are the Guild hunts, where both Wrizz and Soren can drop various amounts of diamond chests which can give you 10, 30 or even 100 diamonds each. While this amount isn't massive, it's a steady way to earn diamonds every day.
Bounties can give you various amounts of diamonds daily. The number of diamonds depends on the bounty rarity and bounty type (single or team).
You can also spend 50 diamonds to refresh bounties (single bounties only, team bounties won't refresh!). This can help you to acquire some additional diamonds. Refresh function works best with a high level of bounties (e.g. 6 or 7) because low-level bounties won't appear on your bounties list.
You can get some number of diamonds and other rewards by doing Voyage and Peaks of Time missions. Usually the aren't massive, but still decent numbers. Treasures from each Peaks mission you can acquire just once, but Voyage will get you new loot after its reset!
You can get some diamonds for free every day, the amount is based on your position in Arena of Heroes leaderboard.
The person that holds 1st position on the leaderboard gets 80 diamonds, lower-ranked players get smaller amounts of diamonds. Moreover, you will get 10 times the diamonds you get daily every 2 weeks when Arena of Heroes 2 weeks season ends. Remember to do your battles on Arena daily to get more diamonds every day.
Besides other fabulous prizes, you can also acquire diamonds for beating floors of the King's tower, as well as completing campaign quests by conquering specific floors (e.g. 100, 110 etc.).
While the reward for a single floor isn't outstanding you can often beat several of them in a row, earning a substantial number of diamonds.
By fighting the end boss of each Labyrinth floor you can win some diamonds, moreover, reward chest at the end of each floor can give you up to 1000 diamonds, so it's a thing to fight for.
Sometimes among the hostile tiles, you can find a trader who can sell you gear and it has a reduced price too!
You can buy diamonds in Gift shop and as a part of either daily, weekly, or monthly gift sets.