I made a research about diamond rarity last month.
According to research, the results:
-Diamond is more common at Y:12. It's also common at Y:11 too.
-There is 384.2 diamonds in a 100 chunk area.
-1 out of 67 block is diamond ore (below Y:16)
-%0.014 of blocks are diamond ore (below Y:16)
Detailed research results:
(L:Layer, D:Diamond)
L1 10D
L2 17D
L3 9D
L4 13D
L5 24D
L6 28D
L7 61D
L8 51D
L9 38D
L10 26D
L11 46D
L12 50D
L13 24D
L14 11D
L15 2D
Total 460
L1 6D
L2 20D
L3 25D
L4 26D
L5 21D
L6 34D
L7 31D
L8 37D
L9 25D
L10 12D
L11 12D
L12 48D
L13 38D
L14 12D
L15 0D
Total 347
L1 3D
L2 11D
L3 21D
L4 25D
L5 17D
L6 35D
L7 43D
L8 41D
L9 46D
L10 36D
L11 30D
L12 23D
L13 20D
L14 8D
L15 4D
Total 363
L1 4D
L2 12D
L3 24D
L4 20D
L5 38D
L6 46D
L7 40D
L8 35D
L9 32D
L10 28D
L11 47D
L12 34D
L13 6D
L14 5D
L15 4D
Total 375
L1 4D
L2 10D
L3 16D
L4 31D
L5 43D
L6 40D
L7 39D
L8 32D
L9 24D
L10 25D
L11 36D
L12 37D
L13 23D
L14 14D
L15 2D
Total 376
Average 384.2
Research made in 10×10 chunk area, 5 different places in overworld in a normal Minecraft world.
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