Simplify the following text for a Grade 4 native English-speaking person.
Original: Nearly half are still at Rano Raraku , the main moai quarry , but hundreds were transported from there and set on stone platforms called ahu around the island 's perimeter .
Simplified: Each moai is made out of one large stone but some have an extra stone on top of the head . Most were made from the volcanic rock in the Rano Raraku area of the island .