caves. deep underground caves. branch mining is too boring and takes forever with little to no fun involved.
if you fully explored the cave deeply, you have millions of other caves you can explore. that's how i get my diamonds.
The issue with solid stone branching, is that you have to make sure you make your branches long enough. Anything less than at least 32 blocks is generally going to be pointless, but then again, after that point travel time starts becoming a pain, unless you either use Nether portals or teleportation mods. Branching is also only really painfully boring if you're using stone tools, and/or aren't making your branches long enough, so you don't find anything.
5-12 is the best places to find diamonds though if you stay on layer 12 you are not likely to come across any lava pools. What I do is make a shaft at least 5x5 all the way down to the bedrock then mine around level 10-12 by branching. When it comes to branching I make the "arms" 3 spaces apart because you will rarely find that diamond stay in one spot and goes strait up. It will almost always go side to side, so you have a good chance of finding it.
(Tip) always mine all the way around a diamond vein if you find one so you don't fall into lava while mining it.)
How do you know the layers? I don't know how to find layers. Can you teach me?
Press your F3 key it displays the level you are on as the y cooridinate on the lefthand side of your screen, for instance it might say...
y: 11.000 (feet pos,12.620 eyes pos.)
This is telling you that your feet are in level 11 (standing on top of level 10) and your eyes are at level 12.62, this is the height you want to mine at to stay above the lava pools that form on levels 1-10. As you mine forward and find lava just pour your water bucket on it to turn the lava into obsidian, scoop your water back up with the bucket and keep going forward.
Once you have a central shaft going for a ways start digging 1x2 tunnels on both sides of it every two blocks this is called branch mining, it exposes the faces of all the other blocks that you are not digging so that you don't miss anything.
Digging anywhere between layer 5 and layer 12 will yield identical results as is shown in this graph of where diamonds are actually located in the game...
Since the distribution of diamonds is very nearly identical the only thing that really matters is which layers are the easiest to mine.
On layers 1-10 the game fills all natural air spaces (caves) with lava by default, so anytime you dig below layer 11 you stand a much higher chance of running into the side of a lava pool. Dealing with lava flows is dangerous and time consuming and should be avoided where at all possible in order to achieve maximum productivity.
Many people think that more diamonds are generated near lava, this is not true, it only seems that way because when exploring lava caves diamonds are sometimes exposed on the surface of the cave. Here are two pictures, one shows where the lava is located while the other shows the diamonds in the same area. Do you see any clusters of diamonds bunching up near the lava?
Diamonds are distributed totally randomly throughout the entire map, the presence or absence of lava has no effect on their generation. (In fact lava actually replaces diamonds that might have been generated where the lava pool was formed.)
All of which leads us back to the question, where is the best and most labor efficient place to mine diamonds? The answer is quite obvious even though many players out of shear contrariness refuse to accept it as true. You should mine with your feet in layer 11 and your eyes at layer 12.62, you will achieve statistically identical results to what you would mining at any other level while staying at the surface level of the lava pools.
Mining at layer 11 & 12 allows you to simply use a water bucket to turn the surface layer of the lava to obsidian as you move forward and to explore the interior of the lava caves as they are revealed. Digging lower forces you to go around many lava pools and deal with lava flows released by accident while not returning any increase in diamonds found for your efforts.
Branch mining (It isn't called strip mining! Strip mining is when you "strip" away the surface of the world! Like what most people call quarries.) is the most efficient method as it reveals the most block surfaces for the least amount of blocks mined. There are several different methods favored by people for branch mining as to how to create your tunnel configuration but they all have one thing in common.
That being that for each two blocks mined you reveal the faces of six additional blocks, so digging parallel 1x2 tunnel branches two blocks apart from each other reveals the faces of all the blocks in your two layers along with many of those in the layers above and below you. There is only one other method of mining that will yield comparable results to branch mining when comparing the number of diamonds found per block mined and that is cave mining.
Finding caves that go down to the 11-15 layers and mining exposed diamonds does result in collecting a few diamonds that are already exposed on the surface without having to mine many blocks. However it is very hit or miss to locate them and not that many large caves are found that go all the way down to the lava level. Also keep in mind that while branch mining at 11-12 you will still find these same caves and can and should explore them thoroughly up to level 15 in case they have any exposed diamonds.
This question has been asked and answered repeatedly on this forum and even though the only correct answer is to branch mine on layers 11 and 12 many people have their own pet theories. The fact that these theories have no objective evidence to support them does not matter to them, they have concluded based on their own personal intuition that their method is superior.
It isn't, but you you will never convince them of this so these threads get made time and time again and will continue to be made, always getting many different answers. There are some people that simply can't be convinced of the truth.
Despite most people who say level 12, I beg to differ.
I usually find payloads at Level 8. Lava is a danger, but the reward is better. I just dig straight down(I haven't fallen in lava ONCE yet), and Branch Mine from there. I usually get around 10 diamonds per five minutes. On a server though, I'd recommend finding new chunks to mine in.
Technically speaking mine craft wiki has a chart showing the best layers for diamond digging i believe it was layer 6-11 that faired very well btw the most diamonds generally spawn more in mountain ranges.