Diamonds are found in the bottom layers of the map. Not at only bedrock level, but also a bit above that. The best way to find a diamond is to just, when finding caves, go down the routes that look promising. Try always going down hallways instead of up, because it cannot be found a certain elevation.
Make sure that when stepping off of a raised platform, to look behind you to make sure you didn’t miss any. I have, many times, missed diamonds because I jumped off and didn’t look at the block where I stepped. It is often found near lava, but not always.When you find a diamond, my advice is to dig out around it first, especially the bottom. Diamonds are rare, and the worst thing is to dig out a block, only for it to fall into lava beneath it. You are most likely to find diamonds in a ravine, and sometimes caves if you’re lucky.
Well, that is dependant on what rules there are. You could say that xray the best, however I believe this ruins the Minecraft experience and should be banable on every server. I believe the best way to mine diamonds is to slowly mine down in a staircase to the y coord 12, and then mine in a straight line. This seems almost “nooby”, however, I believe it is the safest and saves you from mining into lava.
The most efficient method to mine diamonds is to use a Fortune III pickaxe, as it allows you to get up to 4 diamonds from a single ore block. I will, however, assume the question intended to ask also how to find the diamond ores, in which case the most efficient and widespread method is strip mining (either by making long tunnels with 1×1 holes on the sides or long tunnels with smaller tunnels on each side), all of this of course below Y = 16. There are other valid methods to obtain diamonds and other loot, such as end busting, but they don’t involve any mining so I will not get into detail.
In early game mining for diamonds is easiest by caving since you don’t have the necessary tools to be constantly mining blocks. After getting your first five diamonds, you are able to craft your first diamond pickaxe as well as an enchantment table. Using an enchanted diamond pickaxe greatly improves durability and increases the amount of blocks you can break. Once you are at this step I would recommend strip mining at y=12, to avoid lava pools since they form at y=11, 3 blocks apart mining mining until you think it is too far to make the journey back in a reasonable amount of time. For end game I would recommend efficiency V pickaxe paired with a beacon with haste 2 and just insta-mine every block in the beacons range. This greatly increase blocks broken per second along with amount of diamonds found.
Diamond can be found anywhere beneath layer 16, but is most common in layers 5-12. So the method for finding diamonds is simply to dig around those levels and look for veins of ores and lava as well, as diamonds sometimes spawn near lava sources.