|Acid dissolve rate
Clone particles basically make endless copies of the first other particle they make contact with.
These particles have a special variable (ctype) for remembering what the last thing they touched was, and this is preserved in stamps/saves, meaning you can transport working clone particles into tight spaces if you work carefully. If you draw over any clone type with the brush, it will clone the element you had selected instead.
Clone particles are also transparent to photons, meaning they act like glass to them. This is useful for making lasers.