Mining is changed from vanilla Minecraft through the HiddenOre plugin. In order to prevent X-ray cheating there were no ores in the map on generation. Rather they spawn one at a time when you break Stone, Terracotta, Netherrack, or similar underground material during your mining. The only exceptions are Diamonds which spawn in veins below y=16, and Emeralds which were accidentally pregenerated.
To spawn as many ores as possible in a chunk, don't place torches (use GammaBright) or any other blocks, and always dig gravel/sand from top to bottom. In order to mine without planting torches it would be recommended that one should download the mod GammaBright, which is allowed on the server and allows for the light level to appear as if it were daylight. This mod does not change ore generation however.
While mining, there's a random chance to spawn a Diamond "vein". These veins are shaped like discs rotated in 3d space (like a Flying Saucer or medicine pill), and more Diamonds randomly spawn within them while mining.
You should always mine the ore using Silk Touch so you can process it in an Ore Smelter to get 3 Diamonds per Diamond ore (for comparison, with Fortune III you get only 2.2 Diamonds on average).
Due to a configuration oversight, Emerald ore got pregenerated on world creation. They can be found in the
Extreme Hills + biome in bunches of 1 to 10 ore blocks (whereas in vanilla they only spawn as single blocks and also in "normal"
Extreme Hills). They are the only ore that cannot be spawned in using HiddenOre, since they are used as exp storage.