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Get into geeky gifts that glow

Black Light World

Uranium marbles glow with a greenish hue under ultraviolet light, but they're said to be safe — despite the radioactive sticker on the container.

Glow-in-the-dark uranium marbles have emerged as the top Science Geek Gift of 2011, but you don't have to go radioactive to get that greenish glow.

To be sure, there's something slightly subversive about marbles that are slightly radioactive.  "Definitely geeky, but non-geeks would also love them because they glow and have a risk factor appeal," one commenter wrote.

That's probably a big reason why the suggestion from Richard-1971294 won out over Joel Davis' Star Trek pizza cutter in this year's 10th annual Science Geek Gift roundup.

The totally unscientific tally was close: The margin of victory was less than 50 votes out of more than 3,000 cast. But the green glow of victory means that Richard is eligible to receive a pile of geek-friendly books, including "The Cult of Lego," "Science Ink," "The Physics Book" and "The Case for Pluto." Because Joel came so close, I'm sending him an autographed copy of "The Case for Pluto" as well.

Now, about that uranium: In the old days, pigments containing uranium used to be found in things ranging from ceramic tiles to dinnerware and glassware. Today, uranium isn't used as a coloring agent, but probably not for the reason you'd suppose. Natural, unprocessed uranium isn't all that radioactive — but because it's a heavy metal, it's as toxic as lead. And we all know what happened to lead paint. On the Health Physics Society website, Washington State University's Ron Kathren says "chemical toxicity is the overriding consideration" when it comes to limiting the use of natural uranium.

Uranium marbles, which glow green under ultraviolet light, are still available from Black Light World as well as eBay vendors. If you're serious about the nuclear option — for example, in the form of a spinthariscope toy or a chunk of trinitite — you'll want to check out United Nuclear's wares as well.

A healthier glow
The health risks of radioactive inks and paints have been known since the 1920s, due to the illnesses suffered by the "Radium Girls" who painted the dials on glow-in-the-dark watches. Today, few manufactured items make use of radioluminescence, which involves converting radioactive emissions into visible light. (Exceptions include some types of watch dials, keychains and gunsights that glow due to paints containing tritium or promethium rather than radium.) Virtually all of the glow-in-the-dark items you see today take advantage of electroluminescence, chemiluminescence or photoluminescence.

Electroluminescence is behind the greenish glow in pushbutton timepieces such as Timex's Indiglo line. Chemiluminescence relies on a chemical process — for example, the mixing of chemicals in a glow stick. Photoluminescence involves "charging up" a chemically treated object such as a glow-in-the-dark Godzilla by shining a light on it.

The key substances in most glow-in-the-dark items are phosphors, chemical compounds that are good at taking in energy and emitting it as light. Zinc sulfide and strontium aluminate are the most commonly used glow-in-the-dark ingredients, and new glow-in-the-dark compounds continue to be developed. They're relatively safe: That's why you see so many kids' toys that glow in the dark, as well as these geekier items:

You can set your own environment aglow with phosphorescent paint from ThinkGeek or United Nuclear. For the final frontier in glow-in-the-dark geekery, check out this ghostly green space shuttle at MakerBot's website. If it's bioluminescence you're into, GloFish has been offering fluorescent fish for years, but don't look for glowing kittens or puppies to enter the market anytime soon. In fact, ethical debates over genetically altered organisms like glow-in-the-dark zebrafish have been raging for years. The prudent product for your kids might be a glow-in-the-dark coloring book that teaches them about totally natural bioluminescence.

Speaking of bioluminescence...

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You don't need to buy me a present. All I ask is that you connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.