Remember, Minecraft for 3DS and WiiU are very old versions. Look for diamonds at y-level 11-12. Just like the golden days of Minecraft.
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Modern Minecraft made some slightly annoying changes to the game in my opinion terrain wise When I played Minecraft it was PC v1.2.5 and Pocket Edition alpha and beta. which didn't have a nether till I was about 16 or 17 years old.