The setting of William Golding's Lord of the Flies is a tropical island, so it is not surprising that a shell becomes one of the primary symbols Golding uses in this novel.
Ralph is the first to see the shell, but Piggy is the first to recognize its significance. Ralph sees that "something creamy lay among the ferny weeds," and he retrieves it. Piggy recognizes it as a conch shell and teaches Ralph how to blow it. For Ralph, the conch is something new and fun to play with, but from the beginning Piggy understands its importance.
The shell has the ability to communicate; when Ralph blows the shell, all the other boys gather. During the next meeting, the shell also becomes the symbol of order. Ralph says, "I’ll give the conch to the next person to speak. He can hold it when he’s speaking.” Though the group routinely ignores Piggy when he holds the conch, it is nevertheless the primary symbol of order and civility for the boys.
Like all shells, though, the shell begins to fade and erode from the effects of sun, sand, salt, and water. This parallels the gradual erosion of civility and order on the island. After Jack steals Piggy's glasses, Piggy has finally had enough of being mistreated and takes the conch up the mountain, led by Ralph because he can barely see. When they arrive, Ralph takes the conch from Piggy and blows it to call an assembly.
Piggy demands to know “Which is better—to have rules and agree, or to hunt and kill?” He is shouted down by the savages in the fort who eventually answer him by dropping a boulder on him and killing him. "[T]he conch exploded into a thousand white fragments and ceased to exist."
When the conch, the primary symbol of order and civilization, is gone, so is any remaining conscience or restraint. Jack commands his tribe to kill Ralph, and they undoubtedly would have done it if they had not been rescued.