Yeah, that's all nice and dandy fellers.
Now to something on topic:
Method A: Build two rock chrushers, feed the first cobble from an igneous extruder, feed the second the gravel out of the first one. A rock crusher has a very low chance of producing diamond when it's crushing gravel. Fair warning, this method is rather inefficient and you'll end up with interdimensional barrels full of sand, it needs a lot of energy, AND let's not forget that you actually need 12 diamods per rock crusher.
Method B: The industrial diamond method. This one's much more involved technically and was mentioned already. You can macerate coal into coal dust, craft it into coal ball by placing 8 coal dust around a flint, compress the ball to get a compressed coal ball, craft it into a coal chunk by placing 8 compressed coal balls around an obsidian. And finally compress that into a diamond. As you can see the process is kinda long and semi-complicated, and if you wanna do it fully automatically, then you might want to use a rock crusher for flint, igneous extruders for cobble and obsidian, several macerators (or an industrial one), autocrafting tables and compressors. This is complicated stuff, and you'd have to produce all the machines extra instead of using the ones in your workshop, So that means more diamond, especially if you're using gregtech recipes. I'd suggest doing this by hand. Also, in case you're not very good with math, 8x8=64, you need a stack of coal for one diamond. Think if this is really worth it to you.
Method C: UU matter, this ones is simple, you only need one autocrafting bench for processing the UU into diamonds, however, you also need that for your iridium, so...
I'll be frank with you, machines for diamonds aren't... worthwhile. It's much easier and faster to just cannon ball(=outrun corruption by using a mining laser set to explosive) trough a dense mystcraft world. Word of advice: mushroom island biomes don't have monsters and also spawn rubies AND sapphires.