Named after Tiny Tim's far, far less popular brother, this is a little bitty cyst of hard, chalky material with a thin coating of viscous slime on it. It must've formed around some foreign object -- I wonder what it is? Actually, since I'm an omniscient Narrator, I know what it is -- it's more correct to say that you wonder what it is.
The green peawee marble has approximately a 50% chance of dropping. The brown crock marble will drop approximately 25% of the time. The trend of halving probability appears to occur as you go down the list.
