When does the conch get shattered in Lord of the Flies?

It really depends on which circumstance you're looking for...

The first time the conch is blown when they first get on the island is page 18

"Ralph grasped the idea and hit the shell with air from his diaphragm. Immediately the thing sounded. A deep , harsh note boomed under the palms, spread through the intricacies of the forest and echoed back from the pink granite of the mountains."