In this sentence
Plenty of silicon is found on Earth, but it remains a valuable material for the computer industry.
I'm still not sure if the simple subject is plenty or silicon, but I think it's silicon because the second part of the sentence says it.
2 answers
of silicon is a prepsitional phrase
Anonymous is right; "silicon" is the object of the preposition "of" and cannot be the subject.
The subject is "plenty."
The subject is "plenty."