I'm making a modpack that comes with a bunch of trading mods and other things that use emeralds, and I wanted a way to make emeralds easier to get that wasn't overcomplicated or stupidly OP. In the absence of better options, I figured a 1:1 exchange with diamonds made a certain amount of sense; just pop one in the centre of a crafting table and surround it with eight bits of cactus green dye to get your shiny new emerald.
Full Disclosure: This is a terrible mod. I made it in four seconds using MCreator and I have no idea what I'm doing. If you know of a better (and less wildly silly) way to make emeralds more widely available, let me know; I probably just missed it. If you know how to make it so that the recipe uses anything registered in the oredict as dyeGreen, let me know; this only works with Cactus Green itself, which is super annoying.
TOU: Please do absolutely whatever you want, except include this in modpacks you're making money from in any way. I didn't put enough effort into this to have earned an opinion, but I'm pretty sure using it that way is against MCreator's own TOU.