In the Lord of the Flies, the conch is shattered when Piggy dies. Before that, it was held by Ralph, the de facto leader of the boys. What does the conch symbolize?
The conch symbolizes leadership and civilization throughout the story. In the beginning, Ralph uses it as an extension of his power. For example from the first chapter after he is elected.
Ralph smiled and held up the conch for silence.
Ralph uses it throughout the story to hold meetings and even makes the rule that people could only talk if they have the conch
“And another thing. We can’t have everybody talking at once. Well have to have ‘Hands up’ like at school.” He held the conch before his face and glanced round the mouth.
“Then I’ll give him the conch.”
“That’s what this shell’s called. I'll give the conch to the next person to speak. He can hold it when he’s speaking.”
Piggy's death crosses the line from the boys being civilized to completely uncivilized. That is the point of no return. The shell is even described as being more than simply being destroyed.
the conch exploded into a thousand white fragments and ceased to exist.
The conch establishes Piggy's inferiority to the reader early on, providing the basis for his exclusion from the island society, such as physiognomy (judgement of character via appearance), non-standard English ("I can't hardly move"), and freaky ideas that mark him as an outsider -
"better to have rules and agree".
Ralph finds the conch but Piggy tells him how to use it. Piggy neither tries to demonstrate his knowledge by blowing it himself, nor questions the Ralph's leadership as soon as he blows it -
Piggy, too, raised his hand.
Arguably, the conch triggers Golding's chief condemnation of Nazism: Piggy, the outsider, who is ultimately slaughtered.
After the plane crash, the "sound of the conch" summons the whole population towards Ralph. It becomes a powerful symbol of civilization and order once the assembly votes Ralph as the leader.
This concept is furthered by Ralph's use of the conch as a vessel of democratic legitemacy. It governs the boys' meetings because the boy who holds the conch is given the right to speak. Furthermore, the conch is one of the key symbols that marks the inevitable erosion of civilised feeling whilst savagery dominates the boys and Ralph loses his influence as the non-corrupt leader.
As Ralph picks up on the fact that "Things are breaking up", the conch is his first port of call. However, the meeting loses focus at this point in the novel, where in previous meetings the conch had been enough to maintain order while it had still been a novelty.
Jack makes an "obscene" joke about Simon's suggestion of the beast "with one crude expressive syllable", and finally displaces the meeting when he declares "bollocks to the rules!". Although he knows all his efforts will be in vain, Piggy's immediate reaction is "blow the conch, Ralph!". This highlights the importance of the conch not only to the leader that uses it, but to his supporters, despite the fact that no civilised symbol can overcome the savageness of the people (which is, essentially, Golding's message).
As someone with self-respect and a belief in humanity, Ralph's disgust at his own actions leads him to use the conch as a means of comfort. This advances the plot: the conch not only helps him come to terms with his actions, but consolidates his belief that a) what he did was wrong, and b) Jack was nothing more than a savage.
an affectionate reverence for the conch ~ took the conch caressingly with both hands and knelt
When Roger kills Piggy with a boulder, which has also been mentioned previously, the conch shell is crushed. This signifies the complete demise of civilised instinct amongst almost all the boys on the island.