What Kenjara said about chunk size is correct. However I would add the following points.
- Diamond Ore only appears below level 16
- The highest concentration of diamonds occures between levels 5-12 but because of lava lakes there is slightly less in the middle of that (~9)
- There is 1 diamond ore vien generated per chunk.
- An ore vien will have between 3 - 9 diamond ore in it.
- However that vien can be overwritten by other generated structures such as caves which will leave you with a chunk with no ore vien in it.
- Because of this it is possible to mine an entire chunk and still not get any diamonds.
Because of these points and how long it would take to mine an entire's chunk worth of diamond possible blocks there are more effecient methods than mining every block in the range. If you want to be absolutly certain your don't miss it the most effecient method is lane mining. This is the method where you expose the maximum amount of blocks for inspection while mining the minimum amount.
So pretend this is a top down view of a layer of stone
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You would want to mine like this
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So you leave two lanes of unmined stone between each mined lane. This lets you see the contents of both of the unmined lanes by walking down the mined out lanes and reduces the amount of total mining by to a third of what mining a whole chunk would take.