What did Ralph find? How was it used?

Ralph had stopped smiling and was pointing into the lagoon. Something creamy lay
among the ferny weeds.
"A stone."
"No. A shell."
Suddenly Piggy was a-bubble with decorous excitement.
"S'right. It's a shell! I seen one like that before. On someone's back wall. A conch he
called it. He used to blow it and then his mum would come. It's ever so valuable--"

Ralph used the shell to call meetings.