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A few weeks ago, I wrote a column about beach sand. My knowledge in this area is limited, but a reader (who must be a geologist) helped me out. In an e-mail he points out that true sand is made of calcium silicate (crushed rock), while our white sand beaches consist of calcium carbonate (crushed coral).

Calcium carbonate is cementitious, he explains, which means it has the properties of cement. If placed in the ground and wetted intermittently, calcium carbonate tends to revert to solid coral. This has been a problem in older Hawaii golf courses, where greens get hard and balls bounce out of solidified sand traps.

Golfers may not mind this little boost, but the USGA does. Today, all courses certified for tournament play must be built with calcium silicate mixed in the soil, not calcium carbonate. To meet this requirement, golf course developers in Hawaii must import calcium silicate, usually from Australia.

Cementitious. I love that word.

Another interesting word I learned about recently is purple. I knew this color once symbolized royalty, but I didn't know that it has both a marine and a biblical history.

In response to my recent column on murexes and others snails that bore holes, a minister in Minnesota writes that murexes may be mentioned, indirectly, in the Bible. Luke 16:19 says, "There was a certain rich man, which was clothed in purple and fine linen ..." Acts 16:14 mentions a woman named Lydia who was "a seller of purple."

The word purple comes from the Latin word "pupura," which came from the Greek word "porphura," meaning shellfish yielding purple dye. As the pastor suspected, those shellfish were murexes.

Ancient Phoenicians and Egyptians harvested murexes to extract a dye. But each murex yielded only a small amount of color, thereby making purple clothing affordable only to the rich. In biblical times, the minister explains, people killed millions of snails for their purple color, leaving heaps of empty shells on the seashore.

But interestingly, the secretion in these snails is not purple. It's colorless. According to one textbook, this coveted snail secretion had to be "exposed to air and light and properly extracted and dissolved" to turn cloth purple.

Researchers have found no physiological role for the colorless precursors of murex dye, and believe they are the snails' waste products.

I don't think the royalty would be pleased.

Nor was the man pleased who swallowed a jellyfish while snorkeling. A Massachusetts dermatologist (formerly of Kauai) wrote me about a patient who told a peculiar story. While snorkeling along a coral reef in the Caribbean's Virgin Islands, the man swallowed some water. He felt a sharp sensation in his throat but ignored it.

Days later, he had difficulty swallowing and developed about 10 sores in his mouth. Does this ring any bells? the doctor asks. Was it nematocysts? (Stinging cells on jellyfish tentacles.)

I've never heard of this before, but it sounds like this man may have swallowed a tiny jellyfish. Also possible is a strand of stinging limu or a chunk of drifting fire sponge. Whatever it was, though, I'll bet the guy never swallows sea water again.

Speaking of sponges, a reader writes that he found several on a beach and asks how to clean them for display. I have no idea but would like to know. Please write if you have experience with this.

Or if you don't. Your e-mails make my day.