To people who are trying to come up with various complicated explanations for Pica's powers: Best not to think too much and go for a simpler answer instead, like he's simply a rock Logia.
I remember when someone tried to explain that Absalom has a breath Logia. Breaths cannot be seen, which explains his invisibility, and they are flammable and therefore can explode, which explains his Dead Man's Hand. In the end, it turned out to be nothing more than a simple invisibility fruit…
There was also an instance where Califa was believed to have an ability where when she touches something with one hand, she can change someone she touches into the material that hand is touching with her other hand or cover them in it, which explains why Sanji was seemingly encased in glass. In the end, it was simply a bubble/soap fruit... But then, this belief stemmed from a fake spoiler/rumor.