is there any way of getting diamonds without spending real money? I know you can buy them on currency exchange

The most common way is to buy them on the currency exchange. Sometimes during festivals, you can find some diamonds, but usually these are in small amounts, and are often in expensive features that require diamonds, a sum of gold or tokens to play.

This way is a way that no one should EVER DO: Hack into someone(who has diamonds)'s account and take all their diamonds