I have three things that I think symbolize civilization.
The first one is kind of obvious -- it's the conch. The conch symbolizes civilization because it is the thing that is associated with rules. It shows who is allowed to talk and it calls people to meetings.
The second one is Piggy's glasses. I think this symbolizes scientific knowledge. That is a major part of civilization.
Finally, I would say that the signal fire is a symbol. It symbolizes how civilized people think ahead and plan ways to keep themselves safe. The hunters do not maintain the signal fire and, as time goes by, neither does anyone else.