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After reading my column about Hawaiian seaweed last week, a reader wrote: "Interesting about the seaweed Halimeda, which contains calcium and turns to sand. I once thought (when I was not so young) that sand came strictly from water running over rocks and pulverizing them.

"I wondered about the sand all over the world so far from any rocks, and then learned that some sea creatures contribute to sand. Now seaweeds. Have you written a column on the sources of sand on Hawaii's beaches?"

I haven't, but it's a great idea.

First, don't feel bad if you don't know the roles plants and animals play in sand formation. Merriam-Webster doesn't, either. That dictionary defines sand as "a loose granular material that results from the disintegration of rocks."

Expecting some explanation about organic contributions to sand, I read on. But this reference book failed me. It covers glassmaking, the color called sand and even mentions the sands of time, but marine life doesn't get a word.

This is a glaring omission. In tropical and subtropical areas, sand-forming rocks are often rare or absent, yet these areas host some of the world's finest beaches.

Hawaii's white-sand beaches consist partly of the shells and skeletons of marine animals. And not just those of the snails, urchins, corals and shellfish that we can see. The ocean teems with tiny, one-celled animals, called foraminifera, or forams for short. Some of these shelled creatures are so small, they're invisible to the naked eye.

An estimated 4,000 kinds of forams live in the world's oceans. About 40 of these are planktonic, meaning they drift. The rest live on the ocean floor. In some places, forams are so dense that the bottom sediment is made almost completely of their calcium carbonate shells.

Forams species vary in size and color from area to area. This partly explains the variations in textures and colors of some beaches. The famous pink sands of Bermuda get that color from a foram called Homotrema rubrum, which has pink- to red-colored shells.

On some coral reefs, parrotfish add significantly to the sand load by biting off chunks of coral while grazing on algae. Special bones in the fish's throat grind the coral into fine grains, which drop to the reef through the anus.

Decaying calcium-bearing seaweeds, like Halimeda mentioned above, are also important sand suppliers in Hawaii. About 25 species of Halimeda live in the world's oceans. On the reefs of some coral atolls where Halimeda is abundant, these seaweeds are responsible for as much as 90 percent of the sand.

Halimeda is sometimes called the money plant because its flat, round segments resemble coins.

Not all Hawaiian sand is white, of course. The Big Island is famous for its black-sand beaches created by lava. When hot lava flows into cool ocean water, some of the lava shatters.

Currents catch the smaller of these particles and deposit them ashore, making the beaches there black.

The mineral most abundant in Hawaiian lava is olivine, a greenish-yellow substance made of iron and magnesium. Olivine is responsible for the green-tinged sand on some of Hawaii's beaches. Another common lava mineral here is pyroxene. A deep-brown variety of pyroxene turns some beaches dark.

Microscopic seashells, money plants, exploding lava ... There's a lot more to Hawaiian sand than broken rocks.