How has the significance of the conch changed in Chapter 6?
To me, the significance of the conch changes when they argue over whether Jack should be quiet while someone else has the conch. Before, the shell was the symbol of authority and the rules. People didn't always obey it, but they didn't challenge it. Now, Jack is openly saying the conch is unnecessary. So now the significance of the conch is that it is a symbol of the conflict between the hunters and Ralph's group. It's still a symbol of the rules, but it now serves to divide the boys based on whether they obey the rules or not.
In the beginning of the book "The Lord of The Flies" the conch represented civilization and law. Whenever someone held the conch he was to be listened to and allowed to speak. It created a sense of order for the boys with civilized laws. In chapter 6 the society of the boys has begun to splinter. Ralph calls them to an assembly. Ralph does not call the meeting loudly. Instead he whispers to the twins to call the others to the meeting. He holds up the conch in silence.
Eric shares that they have seen the beast with their own eyes, referring to him and his twin, Sam. Jack breaks into the conversation despite not having the conch. He is contesting the laws of the society. He has just disrupted the civilization as the boys had established it. Jack has essentially demonstrated that he will now make his own laws and rules.