The conch shell in "Lord of the Flies" symbolizes order, structure, community and civilization. Initially, the boys use the shell to call and alert each other. This shows that they desire and need to remain together, in a community.
The boys use the conch shell to indicate who has speaking rights at the moment. One boy says what is on his mind while holding the shell and others request it to comment on what was said. This shows the need for structure and order. As the boys become more savage and lose their sense of civilization the shell loses its importance and power. They speak regardless of who is holding it. However, when Ralph discusses the murder of Simon, he holds onto the shell as if just holding it could bring order to the chaos.
When the boys throw rocks at Ralph for trying to blow on the shell, it shows that the boys have lost all respect for structure and civilization. The final jump into savagery is when Roger pushes the boulder down onto Piggy. The fact that the shell was crushed in the attack accentuates the symbolism. After the destruction of the shell, all hope for civility is lost.