1 nov
2014

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Holy Grails
Ten of the World’s Rarest Gemstones
Ten of the World’s Strangest Gemstones & Minerals
World’s Rarest Things
And here’s some super-rare CHEESE









World's Rarest Metals

Let’s suppose you wanted to strike, with a super-rare metal, the ultimate counterfeit-proof coin for your own mini-realm, or craft a superlative one-in-a-trillion ring or jewelry mounting that will take no prisoners. This ingredient needs to be something you’ll never find at your local mall or on late-night infomercials — or, likely, even in a museum.

Ir Rh Re Au Pd Pt Ag
Rarity
To the left you see a logarithmic graph comparing the physical abundances, by weight in the earth’s crust, of what many would recognize to be the seven most precious or “noble” metals. Ir, Rh, Re, Au, Pd, Pt, and Ag stand for iridium, rhodium, rhenium, gold, palladium, platinum, and silver respectively. These figures are at best educated guesses, but according to most sources iridium is the clear winner. It occurs at about three-tenths of a part per billion, a ratio similar to that of a grain of salt to a Clydesdale, making it about twelve times as rare as gold.

The silver shouldn’t surprise anybody, but look at all that platinum we’re wallowing in. It’s more than twice as abundant as gold, though that’s largely offset by its greater difficulty and expense of extraction. Its supply is also pretty dicey, as 80% of it comes from South Africa.

gold bars Before going further, we should probably qualify our rare metal quest with “non-radiological.” The periodic chart arranges elements in rows and columns according to their chemical properties. About 80% are metals. As far as anyone knows, only elements with atomic numbers of 1 through 94 — hydrogen through plutonium — exist in nature1. Anything 95 and above has to come from a reactor or particle accelerator. As of late 2006 physicists at Russia’s Dubna facility had synthesized three atoms of element 118, eka-radon. All of these superheavies are radioactive and many are very short-lived (less than a millisecond for 118), so none would be practical for coinage or jewelry.

Several naturally occurring metals such as francium, polonium, and astatine are also radioactive and ultra-rare. You’ll frequently read that the total astatine supply in the earth’s crust at any given moment is about an ounce. (As a transitory daughter product of uranium, astatine is too rare to survey directly but its abundance can be inferred from uranium’s well known half-life and decay modes.)

Ir Pt Re Au Rh Pd Ag
Density
Precious metals are heavy. Here’s a chart comparing the original seven. Osmium (not graphed, but discussed below) and iridium are the densest known terrestrial substances at 22.59 and 22.56 grams/cm3 respectively. That’s twice the density of lead or about 8 times that of granite. A cube 6 inches on a side (15 cm) of either would weigh as much as an average adult human.

Some of the superheavy elements will probably turn out to be much denser, platinum coin providing any can last long enough and exist in sufficient quantities to be weighed. Physicists speculate that element 108, as of 1997 known officially as hassium, may be almost twice as heavy as iridium. Dr. Burkhard Fricke, an editor of Physics Letters A, suggested in a paper that densities might peak at element 164, provisionally called dvi-lead, at around 46 grams/cm3.

I recall an old Mission Impossible episode in which the crew smuggled a gigantic quantity of platinum out of some country by casting it into a shiny new bumper and installing it onto the front of a car. In view of the metal’s extreme weight, I wondered how such a car might handle.

Re Ir Rh Pt Pd Au Ag
“Tenacity”
To plot a characteristic we could call tenacity, I informally combined the hardness, stiffness, and melting points2 for each of the seven. Gold and silver melt easily and when pure are so soft you can push your fingernail into them. You need to alloy them with a trace of copper or some other metal to make them durable enough for coins or jewelry. Not so with rhenium, iridium, and rhodium. Although rhenium is the hardest and boasts the highest melting point (3186°C or 5767°F), iridium isn’t far behind in those respects and moreover is the stiffest. Aside from being so rare it’s also the most incorruptible metal of any other, resisting all acids including even aqua regia — a bubbling, fuming, 3-to-1 mixture of hydrochloric and nitric acids worthy of any mad scientist.

Many years ago in Hollywood I pitched a science fiction scenario in which its characters used holographically ornamented iridium coins. They would be spectacularly durable. They would also be impossible to counterfeit, since nothing else that’s really usable would be heavy enough. (Only osmium matches that weight, but since it’s similarly rare nothing would be gained. Worse, its powder ignites spontaneously and it readily forms a tetroxide that can be gravely toxic.) The legendary British firm of John Pinches is said to have once struck an iridium medallion.

palladium coin Jewelry makers already wince at the prospect of working in platinum. Because of its high melting point and quick hardening as it cools, it usually requires a centrifugal cast. But iridium poses even graver challenges. Its melting point is 30% higher, and despite its hardness it’s rather brittle and liable to crack if you try to hammer it. Alloying it with a smidgen of platinum would probably boost its resilience without appreciably debasing its value, though other problems remain.

One solution is to powder the iridium as finely as possible and mix in a moist binder to create a paste. You then form that into whatever shape you desire and bake it in a kiln. This is called sintering. The particles will weld themselves together into a mass at temperatures far cooler than the melting point and the binder will cook away. This is how they make tungsten light bulb filaments.

Other possibilities for iridium crafting include carving it like a stone with diamond or cubic boron nitride abrasive, electroplating with one of its many colorful salts dissolved in a liquid, or performing chemical vapor deposition using iridium hexafluoride (IrF6). As of 2009, at least one outfit is marketing an iridium wedding band. (So far mum’s the word on their technique, though my guess is that they’ve gone the carving route.)

IrIridium Prices$ 585 (steady)
RhRhodium Prices$ 1240 (steady)
ReRhenium Prices$ 90.97 (steady)
AuGold Prices$ 1230 (steady)
PdPalladium Prices$ 790 (-5%)
PtPlatimum Prices$ 1270 (-7%)
AgSilver Prices$ 17.50 (-6%)
USD / Troy Ounce (% Monthly Change)
Here are the approximate prices for those seven metals, from top to bottom in order of increasing physical abundance, plotted logarithmically, last updated in mid October 20143. Though iridium is the scarcest, its price is modest because its utilities are minor and people don’t crave it on an emotional level like they do gold and platinum. All it needs is some good marketing. One selling point might be that most if not all mined iridium ultimately comes from meteorites.

Rhodium has really run amok. Until 1985 it never traded above $1000, but it rose to $5350 in 1991, sunk to $183 in 1997, then spiked to $10,000 in mid 2008. Analysts cite the boom in the use of rhodium for automobile catalytic converters in the late 1980s combined with chronic work stoppages at the South African mines where most of it comes from. Aside from the converters, rhodium serves to harden platinum and palladium and appears frequently in jewelry, especially as a plating over white gold (which raises the question of why they’d bother with the gold at all if you never get to see any).

The Guinness Book of World Records presented Paul McCartney with a rhodium-plated disc in 1979. Here’s an uncommonly enterprising mint [Link broken as of Jamuary 2014 — will keep checking] that now sells solid 1-gram (about 31 to the troy ounce) rhodium medallions, though since the metal is hard and brittle like iridium and thus murder to work with you’ll need to cough up $35 or so over its bullion price.

LavoisierSo... what’s all this business about $1 billion per troy ounce? Are there metals far scarcer than iridium and enormously more expensive than rhodium — while at the same time, non-radioactive or very, very nearly so? As it happens, yes.

Each element comes in varieties called isotopes whose atoms differ in the number of neutrons in their nuclei. You’ve probably heard of uranium-235 and, well, the polonium-210 that did in former KGB officer Alexander Litvinenko. Both are at least moderately radioactive and thus off limits for our quest. But there are all sorts of stable4 isotopes, too. For example, silver comes in two of them, 107 and 109. Their natural proportions are 51.85% and 48.15% respectively. Gold and rhodium are rather unusual in that they occur in only one stable isotope each, gold-197 and rhodium-103. Tin offers the most, ten.

So what you’re looking for is an element that’s extremely scarce in parts per billion, and an isotope of it that’s of such a tiny proportion that the product of both numbers is the smallest of any earthly substance.

Osmium comes in seven stable isotopes, and among them osmium-184 is the rarest at 0.02%. That times the element’s 1.8 parts per billion equals about a half part per trillion. But as mentioned above, osmium’s not the nicest stuff to deal with. Plus, although you can correct me if I'm wrong, no one seems to have a creditable price for 184Os.

For a far more serviceable candidate we don’t have to look far. Platinum comes in stable isotopes 190, 192, 194, 195, 196, and 198. Among those, the scarcest is 190, whose natural occurrence is 0.014%. If platinum as a whole exists at 7.5 ppb in the earth’s crust, 190Pt would be 0.014% of that: 0.00105 ppb or about one part per trillion. Therefore, isotopically pure platinum-190 is the most precious metal in the world.

Robert A. Freitas Jr., author of “Tangible Nanomoney” in Issue 2 of the Nanotechnology Industries Newsletter, speculates a figure for 190Pt of $1,347,960 per gram for 4.19% enrichment. This would come out to $32 million per gram in its pure state, or about $1 billion per troy ounce.

Ten of the World's Rarest Gemstones
World’s Rarest Things
Today’s Date in a Kazillion Languages
Asteroid Facts
Bookworm Atypical
Text © Peter Blinn


Acknowledgements

The Coin Page
Engelhard Industrial Bullion Prices
Index of Isotope Products, Oak Ridge National Laboratory
Platinum Today by Johnson Matthey Precious Metals Marketing
Webelements.com Scholar Edition by Mark Winter, University of Sheffield, UK.

Notes:
  1. Historically this only extended to uranium but nowadays we know of naturally occurring neptunium and plutonium. The mineral muromontite manages to reflect internally some of the particles from the decay of its uranium content, producing plutonium (element 94) in detectable traces. Through similar means there exist traces of neptunium (element 93) in uranium ore and perhaps elsewhere.
  2. For this I’m scaling the Mohs hardness, the Young’s modulus, and the melting point (in Kelvins) to the same proportions and then combining them.
  3. As they say, for informational purposes only.
  4. I’m considering any isotope with a half-life exceeding a billion years or so to be stable.
Crustal abundances shown represent logarithmic averages of values published by the CRC Handbook, Tables of Physical & Chemical Constants by Kaye and Laby, Chemistry of the Elements by Earnshaw and Greenwood, and A Handbook of Physical Constants (American Geophysical Union).

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