et’s suppose you wanted to strike, with a super-rare metal, the ultimate counterfeit-proof coin for your own minirealm. 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.
To the left we have a logarithmic chart comparing the physical abundances, by weight in the earth’s crust, of what many would recognize to be the eight most precious or “noble” metals. Ir, Rh, Re, Os, Pd, Au, Pt, and Ag stand for iridium, rhodium, rhenium, osmium, palladium, gold, 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.
That 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.
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. During the early 2000s physicists at Russia’s Dubna facility began generating element 118, oganesson, by fusing calcium and californium one atom at a time. All superheavies are radioactive and many are very short-lived (less than a millisecond for oganesson), 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 products of radioactive decay, astatine and the like are too scarce and ephemeral to tally directly but their abundances can be inferred from their parent elements’ well-known half-lives and decay profiles.
Precious metals are heavy. Just below is a chart comparing the original eight. Iridium and osmium are the densest known terrestrial substances at 22.56 and 22.59 grams per cubic centimeter. 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, providing any can last long enough and exist in sufficient quantities to be weighed. Physicists speculate element 108, 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/cc.
Naturally we want this stuff to be invulnerable, relatively speaking. To plot what we might call obstinance, I informally combined the hardness, stiffness, and melting points2 for each of the eight. Gold and silver melt easily and when absolutely pure are so soft you can push your thumbnail 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 filmland I pitched a science fiction scenario whose characters used holographically ornamented iridium coins. They would be spectacularly resistant to wear. They would also be impossible to counterfeit, since nothing else that’s really usable would be heavy enough. The only other metal challenging its weight is osmium, but as it’s similarly rare nothing would be gained. Osmium’s machine shop properties are even more of a problem. Its powder ignites spontaneously and readily forms a liquid tetroxide that can be gravely toxic.
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 when you hammer it.
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 cooler than the normal 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 (IrF 6). Since 2009 at least one outfit has been 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.)
Supply and demand
As of the most recent mid-month, here are the approximate exchange rates3 for those metals.
Though iridium is the scarcest, its price is modest because its utilities are minor and the public doesn’t crave it on an emotional level as it does 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, which contain 300-1500 times as much iridium as the earth’s native crust.
Rhodium has really run amok. Until 1985 it never traded above $1000, but it rose to $5350 in 1991, sank to $183 in 1997, then spiked to $10,000 in mid 2008. Analysts cited 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.
The Guinness Book of World Records presented Paul McCartney with a rhodium-plated disc in 1979. In 2009 a mint founded by the late Eitan Cohen designed and began striking 1-gram rhodium medallions and offering them for sale at thirty-odd dollars over bullion value on rhodiumcoin.com. It was a valiant and sincere effort, but the company faced quality control issues (like iridium, rhodium in solid chunks is murder to work with), fell hopelessly behind in processing its orders, and finally went dark sometime in 2013.
So what’s all this business about $3.6 billion per troy ounce? Are there metals far scarcer than iridium and enormously more expensive than gold — 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%. 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.
smium 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.
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.
As of this writing, a well-known chemical supply firm now lists platinum-190 at a 4.19% enrichment for $4890 per milligram with a delivery of 6-8 weeks. Mathematically this converts to $116,706,444 per gram in its pure state5, or about… $3.6 billion per troy ounce.
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.
5. Not including the actual expense of refining it to that
Medical Isotopes, Inc.
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.