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.
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.
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 procured three
or four atoms of element 118, eka-radon, by fusing calcium and californium.
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
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.)
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
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 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
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.
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. The legendary British firm of John
Pinches is said to have once struck an iridium medallion. The only other metal challenging its weight is osmium, 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.
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
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
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.)
As of the most recent mid-month, here is a logarithmic plot of the approximate prices for those seven metals, from top to bottom in
order of increasing physical abundance.
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.
|Ir||$ 475 (↓ 15%)|
|Rh||$ 870 (↓ 12%)|
|Re||$ 85.53 (steady)|
|Au||$ 1120 (↓ 2%)|
|Pd||$ 616 (↓ 6%)|
|Pt||$ 991 (↓ 3%)|
|Ag||$ 15.55 (↑ 4%)|
|USD / Troy Ounce (% Monthly Change)
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 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 (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. 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. You can find Cohen Mint items on eBay
now as collectables. A brokerage has the rhodiumcoin.com domain up for sale at
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.
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
Ten of the World's Rarest Gemstones
World’s Rarest Things
Today’s Date in a Kazillion Languages
Text © Peter Blinn
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.
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).
- 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.
- 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.
- As they say, for informational purposes only.
- I’m considering any isotope with a half-life exceeding a billion years or so to be stable.
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