The 'Mystery Arrows' from 'Mine & Blade Battlegear 2' will, if they hit a player, spawn a diamond. This happens even if they're shot from a dispenser or such.
They are also easy to craft, requiring only a cookie and a arrow. Cookies can be made in bulk just by farming wheat and cocoa beans, and arrows are available en-mass from mob farms.
This means that you can easily get a lot of diamonds early on.

