HiddenOre

HiddenOre is a plugin (source) which removes all natural ore spawning, and instead spawns ores into the world as you mine. It works in tandem with Orebfuscator to make life more difficult for x-rayers.

Mechanics
Every time you break a block, the server's HiddenOre config is checked to see if that block has any drops set. If it does, a die is rolled against the drop's percentage, and if successful the drop is spawned into the world. Ore spawn chances are dependent on y-level, with probabilities roughly corresponding to vanilla.

Both items and ore blocks can generate as a result of HiddenOre. Typically, ore blocks will spawn into the world around you as you mine. As HiddenOre removes all vanilla ore generation, this is the only way to get ores.

HiddenOre has many checks in place to prevent gaming the system, for example by placing and re-breaking the same block.

Veins


Some ores will additionally spawn as veins. Finding one ore in a vein indicates that other ore of that type is nearby as well. A message is displayed every time you find an ore in a vein.

Veins are randomly generated using simplex noise and are generally a disc rotated in 3d space. In most civ servers, diamonds spawn in veins, and potentially other ores as well.

Strategies
Mining a long tunnel is a good way to find veins. If a player already possess resources, one of fastest ways to mine is using a haste II beacon and efficiency V silk touch pickaxes. If a player has a minimap mod, it can be useful to waypoint or otherwise mark diamonds to discern the shape and center of the vein.

Fossils
Mining stone blocks has a random chance to drop a fossil. Fossils are a lored Prismarine Shard. They can be cracked by using the "Ore Smelter" Factory for random, albeit unlikely, treasure.