Diamonds are a valuable in-game currency in Runes of Magic, and can be used to purchase a variety of items, including mounts, pets, and gear. There are a few different ways to buy diamonds in Runes of Magic, but the most common is to purchase them from other players through the in-game Auction House.
Another way to obtain diamonds is to complete quests and achievements. Some quests and achievements will reward you with diamonds, and you can also earn diamonds by participating in PvP battles and winning.