The conch is found in chapter 1. It is first found by Ralph but Piggy is the one who realizes that it its a conch. He states "he used to blow it and then his mum would come. It's ever so valuable." This shows how intelligent Piggy is.
Once Ralph used the conch to summon the other children on the island, the came to a group decision to make the conch a talking stick. The conch now symbolizes rules and order, "powerfully, there was the conch."
As the boys become more savage and begin to disobey or disrespect the rules the conch becomes less valuable. For example when Piggy tries to enforce the rules but Jack argues back, "The conch doesn't count... so you shut up."
Now the boys are fully savage the conch has been described as "fragile" this shows that as the conch begins to become more fragile the rules and order began to weaken as well.
Jack and his gang break into Ralph and Piggy's hut to steal some valubles. Ralph and Piggy "thought they wanted the conch" however Jackand his gang steal Piggiy's glasses instead. this shows how the conch is now wortless as Jack doesnt want it the same way he doesn't want rules.
In the process of killing Piggy the conch id knocked and broken. Golding says "the conch exploded into a thousand white fragments and ceased to exist." this show that as the conch breaks any connection to civiliation breaks with it.