The default configuration file doesn't generate diamonds. This fixes the default config file, but will not fix an existing config file.
The ores-per-chunk value of the diamond ore is smaller than the average per chunk (min + variation/2).
IMPORTANT: To fix this on your server set the values for the vanilla-diamond exactly like below. The important change here is the ores-per-chunk value changed from 2 to 3.
Code (YAML):
vanilla-diamond:
element-type
: 'DIAMOND_ORE'
ores-per-chunk
: 3
vertical-region:
max-y
: 16
min-y
: 0
ores-per-vein:
min
: 2
variation
: 3
replaces
:
['STONE'
]