Diamonds are one of the strongest currencies in Hay Day, if not the strongest. This is acquired for free in small amounts that you can accumulate to buy important things in the in-game store. One of the ways to earn diamonds is by growing wheat. By harvesting this grain, you will get diamonds as a reward. Taking good care of farm animals and participating in events will also earn you good diamond rewards.
But if you want to get lots of diamonds, what is most recommended is to top up with your credit or debit card so you can buy everything you need.