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The biggest mineral show in the world gets underway in Tucson, Arizona this weekend. More than 40,000 people are expected to gawk at gemstones and exotic minerals, cubes of bright orange wulfenite and jagged needles of white natrolite.

One of the most exciting things that can happen at this show is when someone unveils a mineral that science has never seen before. Collectors will pay tens of thousands of dollars for such rare finds. And even though people have been digging around for millennia, they are still discovering weird new things underground. NPR's Nell Boyce has more.

NELL BOYCE, reporting:

Go through a metal door on the side of a small mountain and you've entered one of the richest mineral deposits on Earth. It's called the Sterling Hill mine. Geologist Earl Verbeek says that for mineral collectors, this is holy ground.

Mr. EARL VERBEEK (Geologist): They just want to have all these bizarre, rare minerals in their collection, and this is one of the Meccas of the mineral world for them.

BOYCE: Sterling Hill just shows that when people go digging for one thing, they can dig us something totally unexpected. The mine is a museum now. But up until 1986, workers ventured down a half a mile to blast out black zinc ore.

(Soundbite of mining)

They also found a lot of strange stones in all that broken rock. In fact, Sterling Hill, along with a sister mine next door, holds the world record for mineral discoveries: 67 new finds.

Mr. VERBEEK: Some of the minerals here are fabulously rare, I think there are about 31 to 33 of them that still have not been found anywhere else on Earth.

BOYCE: And some of them look spectacular. We walk through tunnels of solid gray rock until we reach the Rainbow Room. Verbeek switches on an ultraviolet lamp and the walls start to throb with neon bright color.

Mr. VERBEEK: It'll take your eyes a minute or two to adjust to the new darkness level.

BOYCE: Wow, it's pretty crazy, I mean, you walk by and it just looks like some rocks and then you turn the light on and all of a sudden it looks like there's bizarre, you know, outer space red and green glowing moss.

Mr. VERBEEK: We typically hear comments like, oh gee, in white light these minerals look like what I'd kick out of my driveway and then you put ultraviolet light on them and kill the overhead lights and they just explode into color. Quite amazing.

BOYCE: Altogether, geologists have found more than 300 minerals in these hills. That's about 10 times what you'd find if you just went digging in your backyard. Now, Peter Heaney says that most people don't understand the difference between a mineral and a plain old rock. He's a geologist at Pennsylvania State University. Heaney says a rock is just a mishmash of minerals. The next time you see a granite kitchen counter, take a close look.

Mr. PETER HEANEY (Professor, Department of Geosciences, at Pennsylvania University): You'll see the outlines of different minerals. They have different colors oftentimes, different shapes. And so it turns out rocks are just things that are made up of lots of different kinds of minerals.

BOYCE: If you could peer inside a mineral, you'd see that its atoms are arranged in an orderly, repeating pattern, like cubes or hexagons. And each mineral is made from a unique combination of elements.

The earth has 92 naturally occurring elements, things like iron, calcium, and sulfur, and they've had billions of years to mix and match. So you'd think there'd be an almost infinite number of minerals. But in fact, geologists have only found about 4,000. You can see most of them at the Smithsonian Institution, in Washington, D.C.

Curator Jeffrey Post leads us past the Hope Diamond, through a side door and into a vault-like room where the keep some of the newest discoveries.

Dr. Jeffrey Post, Ph.D. (Geologist and curator-in-charge, Smithsonian Institute): This is an area we call the Blue Room.

BOYCE: That's because it's painted blue, and the walls are lined with glass cases full of huge, dramatic crystals.

Dr. POST: You look here and see the beautiful colors, the shapes, you know, all of these are natural formations that came from the earth. They're very desirable to collectors. And the prices of some of these have gotten to be quite high. I mean, it's not unusual at all today to see some of the finest mineral specimens selling for more than a hundred-thousand dollars.

BOYCE: Those prices create a big incentive to find new minerals. And Post says that the most amazing finds can make their first appearance at the Tucson show. To see one example, we enter an even more secure vault.

It's a large cut gemstone that shines a deep raspberry-pink. When this mineral showed up at Tucson a couple of years ago, it had just been unearthed in Madagascar. The seller was promoting it as a new color variant of beryl, an old mineral. Then, during the meeting, people realized that this one contained high levels of the rare element, cesium.

Dr. POST: And so that immediately got everybody thinking that this may be a new mineral. And so, of course, the prices went up, zoom, everybody was buying them up, because they're all thinking, Oh, a new mineral, that's great!

BOYCE: This one did turn out to be new, and it's called pezzottaite. Like all new minerals, it had to get the blessing of an International committee known as the Commission on New Minerals and Mineral Names. It reviews around 80 candidates a year; a quarter get rejected because they're just a new form of some old mineral.

Most new minerals don't look that spectacular. Post points to some tiny brown crystals that have grown on a big chunk of pink quartz. It's hubeite, a new mineral from China.

Dr. POST: A lot of times, nowadays, when a new mineral is found, you know, it's a very, you know, little bit of a grunge or smudge on the surface of something else.

BOYCE: Even smudges can be of interest, because many minerals have interesting industrial applications. For example, minerals known as zeolites have an unusual structure that can trap other chemicals. Post says, synthetic zeolites are now used in everything from petroleum processing to cat litter.

Dr. POST: Minerals are the products of nature's own laboratory. And many of these minerals form under conditions that are not either readily accessible in the laboratory, or maybe are conditions that just are so unusual that we wouldn't think to try.

BOYCE: Unusual minerals can also hold clues about what was going on in the earth as it formed long ago. That's one of the reasons why Post is going to be one of the thousands of people in Tucson this week. He's hoping he'll see something that he's never seen before.

Nell Boyce, NPR News.

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