In the skill Science it's not allowed to answer "das Plastik" for the picture with the plastic bottles, but you have to say "der Kunststoff". While in science, people often use the formal term "Kunststoff", in general "Plastik" is much more common. Especially if you see bags full of old plastic bottles.
Hmm, so "das Plastik" is an exception to the rule that nouns ending in -ik are feminine. Possibly because it's a recent loanword?
According to the Duden, die Plastik comes from French, while das Plastik comes from English. That suggests to me that das Plastik is the more recent loan, especially since sculpture obviously predates modern plastic materials.