Some friends and I have a small Horizons server, and one of my first projects was setting up a deep mine-shaft. So far, each level consists of a 9x9x3 central room, with a 1x2x5 tunnel in each wall to a 7x7x3 branch room. Floors are 1 block thick. (So basically clearing out a large amount of material from four chunks with each level.) Levels are connected by a central pillar with ladders. The intent is that the side rooms can be used as 'mining bases' for extended quarrying.
I've hit Void Fog, and haven't found a single diamond. Tons of other stuff, (including random blocks of Hepatizon Bricks!), but no diamonds - which means that the lovely lakes of Obsidian that I made on a couple of floors are now impenetrable obstacles. (I would have made a stack of buckets and done a 'poor man's portal' if I'd suspected this was going to be an issue.)
Bad luck, or did all the new ores suck up the likelihood of finding Diamond, and I should just make a smeltery and be done with it?