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10 of the World’s Rarest Gemstones

There are roughly 200 varieties of natural gemstone known in the world today. Alongside the world’s precious gems (diamond, ruby, sapphire, and emerald) are numerous semi-precious stones, some of which are so incredibly rare that their value outstrips many of the world's most valuable precious gems. Here are a few of the rarest from around the world.

1. TANZANITE // FOUND ONLY IN TANZANIA

 
Tanzanite is a beautiful blue variety of the mineral zoisite, and is so named because it is only found in a small area near the foot of Mount Kilimanjaro in Tanzania. The stone was not discovered in commercial quantities until the 1960s and since then its popularity has grown tremendously, thanks largely to the efforts of Tiffany & Co. Heat-treating tanzanite at very high temperatures can improve the blue coloration, so most gems on the market have been treated in this way, but any tanzanite that has not been heat-treated and has a strong blue color naturally will be of a much higher value. Because it is only found in one small location, the value of tanzanite looks likely to soar over time; once those mines have been emptied there will be no new stones coming onto the market—unless a new source is found.

2. BLACK OPAL // THE DARKER THE BETTER

Daniel Mekis via Wikimedia Commons // CC BY-SA 3.0

 
Opals are usually a creamy-white color and are made special by the rainbow-colored inclusions that reflect the light as the stone is moved. Black opals are much rarer, because almost all of them are found in mines in the Lightning Ridge area of New South Wales in Australia. The darker their background color and brighter the inclusions, the more valuable the stone. One of the most valuable black opals of all time is the "Aurora Australis," which was uncovered in Lightning Ridge in 1938. The 180-carat opal is especially admired due to its large size and intense harlequin coloration; in 2005 it was valued at AUS $1,000,000, or about $763,000 U.S.

3. LARIMAR // ONLY FOUND IN THE DOMINICAN REPUBLIC

Wikimedia Commons // Public domain

 
Larimar is a very rare blue variety of the mineral pectolite and is found in only one small area of the Dominican Republic. This turquoise stone’s name was created by the man who brought the stone to prominence in 1974, Miguel Méndez—he took the first part of his daughter’s name, Larissa, and combined it with the Spanish word for sea, mar, to create the portmanteau larimar. Locals had known of the existence of the stone for generations, because small examples had washed up on the seashore, but it was not until the 1970s that sufficient quantities were found in the ground to open a mine.

4. PARAIBA TOURMALINE // NEON LUSTER

DonGuennie (G-Empire The World Of Gems) via Wikimedia Commons // CC BY-SA 4.0

 
Tourmalines are common in many colors across Brazil, but the Paraiba tourmalines are the only stones with a bright turquoise hue, thanks to their copper content. The very rare gems were discovered in 1987 by determined miner Heitor Dimas Barbosa, who had been driven by a belief that something special lurked under the hills of the Brazilian state of Paraiba. Barbosa was right, and after years of fruitless digging, he finally unearthed a tourmaline of unrivaled neon blue that set the gem market alight. The extremely rare stone (only one stone is mined for every 10,000 diamonds) then became intensely sought-after. In 2003 very similar turquoise-colored tourmalines were found at mines in the mountains of Nigeria and Mozambique, although some say they are not quite as striking as the Paraiba tourmaline.

5. GRANDIDERITE // ONE EXCEPTIONAL EXAMPLE

DonGuennie (G-Empire The World Of Gems) via Wikimedia Commons // CC BY-SA 3.0

 
Grandiderite was first described in 1902 by French mineralogist Alfred Lacroix, who found it in Madagascar and named it in honor of the French explorer Alfred Grandidier, an expert on Madagascan natural history. This extremely rare blue-green mineral has been found in a number of places around the world, but so far only Madagascar and Sri Lanka have produced any gem-quality stones, and these are still extremely scant. The majority of the known stones are translucent, but the most rare, and therefore most valuable, example ever found was transparent. In fact, the stone was initially assumed to be another rare gem, serendibite, because grandiderite of that color and transparency had yet to be seen. The gem was only identified as grandiderite after expert analysis and was subsequently sold for an undisclosed sum. It’s safe to assume that if a gem of similar quality were to be unearthed, its scarcity alone would ensure it fetched an extremely high price.

6. ALEXANDRITE // COLOR-SHIFTING GEM

 
The amazing color-changing stone alexandrite was discovered in 1830 in the Ural Mountains in Russia and named after Russian tsar Alexander II. A variety of chrysoberyl, the stone’s remarkable color-shifting capability makes it especially sought-after: In sunlight the stone looks blue-green, but under incandescent light it becomes red-purple. The degree of color change varies from stone to stone, with some only showing marginal change, but the most valuable are clear stones that demonstrate complete color change.

Although some large examples of the stone have been found (the Smithsonian houses the world’s largest known cut sample of alexandrite at 65.08 carats), the majority are under one carat. This means that the value of a gem under a carat may only be $15,000, but a stone larger than one carat might fetch as much as $70,000 per carat.

7. BENITOITE // STATE GEM OF CALIFORNIA

Pablo Alberto Salguero Quiles via Wikimedia // CC BY-SA 3.0

 
Benitoite is only mined in one small area of California, near the San Benito River (hence the name), but the mine closed for commercial mining in 2006, making this gemstone yet more scarce. The gem was first identified around 1907 by geologist George Louderback and has a deep-blue color that shows especially interesting qualities when caught under UV light, when it glows fluorescent. The gem was named the official state gemstone of California in 1985 in recognition of the fact that, despite it being found in trace quantities in Arkansas as well as Japan and Australia, California is the only place where it can feasibly be mined. Due to the rarity of discovering a good quality benitoite of a reasonable size, it can fetch huge prices on the open market—a well-cut benitoite stone at over 2 carats can fetch more than $10,000 a carat.

8. PAINITE // ONCE THE WORLD’S RAREST GEM

Rob Lavinsky via Wikimedia // CC BY-SA 3.0

 
Painite was first discovered by British gemologist Arthur Charles Davy Pain in 1951 and recognized as a new mineral in 1957. For many years only one specimen of the dark red crystal was in existence, housed at the British Museum in London, making it the world’s rarest gemstone. Later on other specimens were discovered, although by 2004 there were still fewer than two dozen known painite gems. However, in recent years a couple of mines in Myanmar have begun to produce some painite, and there are now said to be over 1000 stones known. The scarcity of this gem has made it extremely valuable and just one carat can fetch more than $60,000.

9. RED BERYL // TINY AND SCARCE

Didier Descouens via Wikimedia Commons // CC BY-SA 3.0

 
Red beryl, also known as bixbite or red emerald, is so rare it is estimated by the Utah Geological Survey that a single such gem is uncovered for every 150,000 gem-quality diamonds. Pure beryl is colorless and only gains its bright hues from impurities in the rock: chromium and vanadium give beryl a green color resulting in an emerald; iron provides a blue or yellow tint creating aquamarine and golden beryl; and manganese adds the deep-red color to create red beryl. Red beryl is only found in Utah, New Mexico, and Mexico, and the majority of examples found are just a few millimeters in length, too small to be cut and faceted for use. Those that have been cut are generally less than a carat in weight, and a red beryl of 2 or 3 carats would be considered exceptional.

10. TAAFFEITE // DISCOVERED BY CHANCE

DonGuennie (G-Empire The World Of Gems) via Wikimedia Commons // CC BY-SA 3.0

 
Austrian-Irish gemologist Count Edward Charles Richard Taaffe bought a box of cut stones from a jeweler in Dublin in the 1940s, thinking he had purchased a collection of spinels. But on closer inspection, he noted that one of the pale mauve gems was not reacting to the light in the same way as the rest of the spinels, so he sent it off to be analyzed. The results revealed that he had discovered a hitherto unknown gemstone—a serendipitous but frustrating situation, since he had discovered a cut gem and had no idea where the mineral naturally occurred. Fortunately, once the new stone had been announced, many other collectors re-examined their own spinel collections and a number of other samples were uncovered. Finally the source of the stone was tracked down to Sri Lanka, although a handful have also been found in Tanzania and China. It is thought that less than 50 examples of taaffeite exist—many of which are housed in geological and private collections, making this gemstone so rare the ordinary public are unlikely to ever encounter it.

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Big Questions
Just How Hot Is Lava?
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iStock

Like the bubbling cheese of a pizza consumed too quickly, lava has been anointed as one of the most scorching substances on Earth. But just how hot is lava? How quickly could it consume your flesh and destroy everything in its path?

You may already know that lava is actually molten rock that oozes or spurts out of volcanoes because of the extreme temperatures found miles deep in the Earth. As the rocks melt, they begin to rise toward the surface. (Lava is typically referred to as magma until it reaches the surface.) As you can imagine, the heat that's needed to melt rock is pretty staggering. Cooler lava—relatively speaking—could be around 570°F, about the same as the inside of your typical pizza oven. On the extreme side, volcanoes can produce lava in excess of 2120°F, according to the United States Geological Survey.

Why is there so much variation? Different environments produce different chemical compositions and minerals that can affect temperature. Lava found in Hawaii from basalt rock, for example, tends to be on the hotter side, while minerals like the ones found near the Pacific Northwest's Mt. Saint Helens could be a few hundred degrees cooler.

After lava has erupted and its temperature begins to lower, it will eventually return to solid rock. Hotter lava flows more quickly—perhaps several feet per minute—and then slows as it cools, sometimes traveling only a couple of feet in a day.

Because moving lava takes its sweet time getting anywhere, there's not much danger. But what if you did, in some tremendously unfortunate circumstance, get exposed to lava—say, by being thrown into a lava pit like a villain in a fantasy film? First, you're unlikely to sink rapidly into it. Lava is three times as dense as water and won't simply move out of the way as quickly. You would, however, burn like a S'more at those temperatures, even if you wouldn't quite melt. It's more likely the radiant heat would singe you before you even made contact with the hypothetical lava lake, or that you'd burst into flames on contact.

Because lava is so super-heated, you might also wonder how researchers are even able to measure its temperature and answer the burning question—how hot is lava, exactly—without destroying their instrumentation. Using a meat thermometer isn't the right move, since the mercury inside would boil while the glass would shatter. Instead, volcanologists use thermocouples, or two wires joined to the same electrical source. A user can measure the resistance of the electricity at the tip and convert it to a readable temperature. Thermocouples are made from ceramic and stainless steel, and both have melting points higher than even the hottest lava. We still don't recommend using them on pizza.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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science
Time Has Only Strengthened These Ancient Roman Walls
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J. P. Oleson

Any seaside structure will erode and eventually crumble into the water below. That’s how things work. Or at least that’s how they usually work. Scientists say the ancient Romans figured out a way to build seawalls that actually got tougher over time. They published their findings in the journal American Mineralogist.

The walls’ astonishing durability is not, itself, news. In the 1st century CE, Pliny the Elder described the phenomenon in his Naturalis Historia, writing that the swell-battered concrete walls became "a single stone mass, impregnable to the waves and every day stronger."

We know that Roman concrete involved a mixture of volcanic ash, lime, seawater, and chunks of volcanic rock—and that combining these ingredients produces a pozzolanic chemical reaction that makes the concrete stronger. But modern cement involves a similar reaction, and our seawalls fall apart like anything else beneath the ocean's corrosive battering ram.

Something else was clearly going on.

To find out what it was, geologists examined samples from walls built between 55 BCE and 115 CE. They used high-powered microscopes and X-ray scanners to peer into the concrete's basic structure, and a technique called raman spectroscopy to identify its ingredients.

Microscope image of crystals in ancient Roman concrete.
Courtesy of Marie Jackson

Their results showed that the pozzolanic reaction during the walls' creation was just one stage of the concrete toughening process. The real magic happened once the walls were built, as they sat soaking in the sea. The saltwater did indeed corrode elements of the concrete—but in doing so, it made room for new crystals to grow, creating even stronger bonds.

"We're looking at a system that's contrary to everything one would not want in cement-based concrete," lead author Marie Jackson, of the University of Utah, said in a statement. It's one "that thrives in open chemical exchange with seawater."

The goal now, Jackson says, is to reproduce the precise recipe and toughen our own building materials. But that might be harder than it sounds.

"Romans were fortunate in the type of rock they had to work with," she says. "They observed that volcanic ash grew cements to produce the tuff. We don't have those rocks in a lot of the world, so there would have to be substitutions made."

We still have a lot to learn from the ancient walls and their long-gone architects. Jackson and her colleagues will continue to pore through Roman texts and the concrete itself, looking for clues to its extraordinary strength.

"The Romans were concerned with this," Jackson says. "If we're going to build in the sea, we should be concerned with it too."

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