Huh, I didn’t know this. I thought that protodermis was created in an attempt to replicate Energized Protodermis.
My thoughts on this (and on protodermis in general) is that nothing in the Matoran Universe is truly pure elemental protodermis, but rather protodermis compounds with other “real” elements.
For example, maybe metallic protodermis is an alloy between copper and protodermis, while the stronger Protosteel is an iron-protodermis combination. (Similar to how “Earth steel” is a combination between iron and carbon)
Following this line of thinking, “rock” protodermis would be a combination of protodermis and other minerals, and liquid protodermis would be a solution of regular water and protodermis.
This theory would explain the vast difference in physical properties between different forms of protodermis; the properties are coming from the other elements, rather than the protodermis. In this case, elemental protodermis doesn’t even have to be a metal at all. This is supported by the fact that Toa of Iron can’t just control anything with protodermis in it.
But, most importantly, this would also explain how Toa are able to control non-protodermis versions of their element.
I’ve gone into this idea before, as well: