Lihzahrd Brick

Explosives bounce off it and nothing below a Picksaw will scratch it, which is the point: the Temple stays sealed until Golem's key is in your hand. Actuators are locked out until Plantera is dead too, so you cannot wire your way through the wall either.

It counts toward a Jungle biome the same way jungle grass does, which sounds better than it is. Without grass there are no jungle plants and no jungle enemy pool. What you do get is an instant biome for a spawn farm, with no waiting for grass to creep across your platforms, and the added biome generally lifts spawn rates.

Strip every last brick out of a world and Plantera starts dropping them, so the material is not something you can permanently lose.