I read that to get diamonds you should mine a whole chunk. what's this "mining a whole chunk" mean?
I would also like to ask that would I get diamond for sure?
What Kenjara said about chunk size is correct. However I would add the following points.
Because of these points and how long it would take to mine an entire's chunk worth of diamond possible blocks there are more efficient methods than mining every block in the range. If you want to be absolutely certain you don't miss it the most efficient 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.
A single chunk is A 16 X 16 area 256 blocks deep. It is probably saying to mine a 16 x 16 area at the level diamonds appear.
As far as I am aware this will not certainly get you diamonds. You are probably better off looking for dirt at the levels diamond exist and mining all of the dirt as diamonds can appear among it.
For diamond mining tactics check out the wiki. https://minecraft.wiki/w/Tutorials/Diamonds
Diamond ore generation has changed since this question was asked. It's also different between Java and Bedrock. Diamonds will only generate from y14 to y-63, and is more likely to generate the lower down you are. This is the same for both versions. I only want to calculate this once, so I will only calculate this for Java Edition of Minecraft.
According to the wiki, Diamond ore will attempt to generate six times per chunk, in blobs anywhere between 0 and 5 ore blocks. Diamond ore can replace stone, granite, diorite, andesite, tuff, and deepslate, but not air or any other type of block. They will also attempt to generate large blobs of size 0 to 23 once in one-nineth of all chunks. If a blob attempts to generate near an air pocket, only part of the blob will generate partially. These numbers are said to be around 50% of the small batch, and 70% of the large batch.
I do not recommend mining any lower than y-59 as you will frequently run into blocks of bedrock, which can be annoying.
For simplicity, let's assume the chunk is solid stone or deepslate. This will eliminate the need for me to incorporate the chances of ore being absent due to air pockets.
If you were to mine out the whole chunk, there is also a fair chance that you will encounter blobs from neighbouring chunks, so I will include that into the equation.
From layers, y14 to y-59 is a total of 74 layers. To clear the chunk you would need to mine 16*16*74 = 18,944 blocks. You would need more than nine whole Netherite pickaxes to mine everything. Or you could use 76 iron picks or 145 stone picks. The average number of ore blocks per blob is (0+1+2+3+4+5)/6 = 2.5, and since it generates six times, there will be an average of 2.5*6 = 15 diamond ore blocks in most chunks. But if you're lucky, there's an 11.1% chance to get a large blob with an average size of (0+1+2 ... 22+23)/24 = 11.5, which could bring your total up to 26.5 ore per chunk. But to average that out even further, there would be 15+11.5/9 = 16.277... (293/18) diamond ore per chunk.
Using the 16.27-block average, there would be 293/18/18944*100 = 0.086% chance of each block being a diamond block. Using this number, there are 16*74 = 1184 exposed blocks for each neighbouring chunk. There are for of them, so there are 1184*4 = 4736 exposed blocks in total. Using our ore:block ratio, 4736*293/18/18944= 4.07 blocks out of all of them will be diamond blocks. (that could include 4 blocks from one blob, or 1 block from four different blobs. For simplicity, we'll just say that this equates to being only 4 extra blocks from neighbouring chunk walls).
If you were to mine out a chunk all the way to the bedrock layer in Java Edition, you will find an average of 15 diamonds per chunk, plus 4 from exposed blocks in neighbouring chunks, for a total of 19 diamonds. And if you're lucky and mine in a chunk with a large blob (1/9 chance of generating), then you have the potential to get an extra 0-23 diamonds.
This calculation assumes you only mine one single chunk. if you mine out multiple chunks right next to each other, then the number of diamonds found o neighbouring chunk walls will vary depending on how many chunks are exposed. Also, mining multiple chunks will increase your odds of finding a large (0- to 23-block) blob. And as mentioned above, I did not incorporate air pockets into this equation.
This tip is using the crawl feature. Mine one side of the outline of a chunk (just the levels that diamonds spawn)(or you can do 9-12 y to be more efficient, then do multiple chunks), as it has a guaranteed number of diamonds that I forget, then on that side, use the crawl feature with a trapdoor or piston and mine like the following graph. # is a mined strip, 0 is a non-mines strip. as you can see, you look at every block in the chunk, while mining only just under a third of the blocks in the chunk or area. This is of my own design though.
#000#000#000#000#000#000# 00#000#000#000#000#000#00 #000#000#000#000#000#000# 00#000#000#000#000#000#00