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You can use the Arcane Bore from Thaumcraft; and I believe that Blood Magic has a ritual that pulls ores from the beneath it, and another ritual that sits above the first one that will mine the ores once they are pulled up. Also, I think Botania has a way of turning smooth stone into ores that you can then mine. That's all I can think of for now.
Diamond ore is usually found between Y 6 to 15 so it's probably best to put the Bore at Y 10 or 11. It will mine a little above and below the level you put it on so that should get you the most common Diamon Ore deposits.