A shell is discovered by Ralph and recognised by Piggy as a "conch". The conch symbolises Law, Order and Civilisation. They begin to utilise it to call meetings on the island between the boys; a form of the theme of order.
S'right. It's a shell! I seen one like that before. On someone's back wall. A conch he called it. He used to blow it and then his mum would come. It's ever so valuable!
Even Jack respects the conch. After he fails to stage a coup, he "laid the conch with great care in the grass at his feet". Instead of throwing it or smashing it, he sets it down carefully. He may not want to play by the rules, but he still respects the rules.
The book shows that rules are only powerful if people agree on them, and that's why Ralph refuses to blow the conch when he knows that things are starting to break down: "If I blow the conch and they don't come back; then we've had it. We shan't keep the fire going. We'll be like animals. We'll never be rescued" (Chapter 5). Due to the fact that he doesn't blow the conch, its power holds.
The Conch shell is also a symbol of democracy, decency and organization. Piggy spends much of the novel cradling it because he needs it to survive. The conch gives boys who do not have a voice a chance to speak. Here in lies the essence of democracy that exists in the boys' homeland: England.
The Conch is a link to the old world of adults. the boys were taught how to be civilised by their parents and teachers and therefore without them, they became savages.
But finally, the conch is broken. Surprise, surprise: it's broken when the brutal Roger pushes a rock over a cliff.When the conch is broken, Jack runs forward screaming that now he can be chief. With no conch, power is once again up for grabs—and Jack is feeling grabby.