Find an extreme hills biome with a cave system and just explore. I tend to find more diamonds in extreme hills than anywhere else. The seed I'm on now I've found about 55 emeralds ( using an fottune enchanted pick axe so the number is actually less).
4 diamonds, 6 gold and gem at x251 y13 z-367. In a big cave system with lots of lava more diamonds, gems and gold around visible from lava to obsidian roads.
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