This plugin provides a possibility to squish several Coal blocks into a one diamond via a special building.

You can use this plugin by just adding the downloaded .jar file into the folder "plugins" of your server directory.

Guide:

1. Build the structure in the Nether World like in the attached images in the end of this plugin description (essential details read at Notes):
2. Drop an Anvil (not chipped or damaged one) to the highest Coal block from the point, located higher than 50 (but less than 100) blocks than the highest Coal block
3. The stack of Coal blocks will disappear, and the Hopper below will acquire one diamond (if a chest is under the Hopper, the diamond will appear in the chest)
   

Notes:

1. Now, you can make Coal blocks not only from coal, but from Charcoal too (recipe: fill all 9 cells in Crafting Table with Charcoal)
2. There must be exactly 5 coal blocks above the Hopper
3. Each coal block must be surrounded by at least 3 obsidian blocks
4. There must be at least 6 Campfires around the Hopper in 2 block radius
5. The chance of turning Anvil into Chipped Anvil after falling in this structure reduced to 10% (after damage the Anvil must be repaired, otherwise the diamond will not appear if you use not new Anvil)
6. If the Anvil is dropped from more than 100 blocks higher than the highest coal block, it will break down completely and no diamond will be produced (60 blocks of height above the highest coal block is recomended)
7. There is 1% chance of getting a Nether Ingot instead of a diamond (I haven’t been lucky enough to get it yet, good luck!)