Minecraft Legends diamonds aren't quite as rare as their original game counterpart, but you're still going to have to go out of your way if you want to gather and use them to upgrade your anti-piglin defences. Unlike the other precious resource, gold, you can actually gather diamonds the same way as any other material using your trusty Allays.
That said, diamonds are only available in certain places on the map. Here, I'll explain how to unlock them as a mineable material, where you need to go in order to locate some, plus a handy trick for getting a few extra.
Minecraft Legends diamonds: How to get them
Before you're ready to get diamonds, you're first going to have to purchase the resource extraction upgrade that lets you mine them. After you destroy your first piglin fortress and the magical device turning the world to night, you'll unlock lots of resource gathering improvements that you can build at the Well of Fate. Press V to access the menu with the gather diamond improvement, then build it for 100 stone and 100 lapis by one of the Improvement Hubs—the four pillars on each side of the well.
This unlocks diamonds as an available resource and lets you use your Allays to mine them. You can find diamond blocks in both the snowy tundra and jagged peaks biomes, though it's a lot easier to locate them in the former than it is in the latter because of terrain. When you find some of these sparkly blocks on the ground, simply use the new gather diamond option that your Allays have to set them mining.
You can also usually collect diamonds from one of the village chests on the map so be sure to hover over the villages with your cursor to check which resources they produce. Return there from time-to-time when the chest is full, and you should be able to gather a few without any extra work.