A. Hawai'i's beaches are made of all kinds of materials, but two experts — Bishop Museum marine biologist Steve Coles and geologist Chuck Blay — agreed that parrotfish aren't major players.

"The amount of sand that parrotfish generate is probably trivial in the total," Coles said.

Blay, who has microscopically studied beach sand on several islands, said the state's beaches are incredibly varied. From the land come grains of the green crystal called olivine and bits of dark basalt rock. From the sea, there are bits of mollusk shell and the tiny shells of one-celled protozoa called foraminifers, plus pieces of coral and of coralline algae, which are rock-hard plants that make up the majority of most Hawaiian reefs.

A lot of the coral is produced by mechanical erosion, such as rocks rolling around on the reef. But a fair amount is also produced by creatures that chew on it, something Blay calls bioerosion.

"Parrotfish are way down the line as far as bioeroding the reef. Animals like clionid sponges and boring clams play a bigger role," he said.

Parrotfish chew on corals, digesting the algae in them and excreting the now-ground-up rock.

Blay said there are beaches in the Islands that are almost entirely basalt, like the black sand beaches of the Big Island, and there are beaches that are almost all shell, coral and coralline algae.

"Every island I've looked at is so different," he said.

He is uncomfortable coming up with statewide averages, but in his recent book, "Kauai's Geologic History," Blay and co-author Robert Siemers note the difference between the sands of east and west Kaua'i.

Volcanic materials make up 39 percent of west side but only 9 percent of east side sands. Reef rock like coralline algae and coral make up 44 percent of west side and 66 percent of east side sands. And shell fragments add up to 12 percent on the west and 20 percent on the east. He was unable to determine the origin of about 5 percent of the sand.