The original website for this mod is:
http://dev.bukkit.org/server-mods/planetoids/

Vein generation algorithm:
Each material in the veins section should have the form:
MATERIAL_NAME-spawnProbability-growthProbability

When creating a planetoid the spawnProbability is used to check for each Block
wether it should be a vein of a particular type or not. These are the generation
0 veins. Note that this probabilities are NOT normalized like the ones for
planets.

Afterwards the generator tries to extend each vein at every side once. The
success probability for this is: growthProbability * (1 / (1 + generation)).
The newly spawned Blocks are parentGeneration + 1 generation Blocks and will be
expanded later.

You can also specify wether single Veins (=Veins that didn't spawn generation 1
veins) should be removed (the default) or not.