Rhodium Is Earth's Rarest and Most Expensive Precious Metal

By: Allison Troutner  | 

rhodium
Although rhodium is rarely seen as jewelry in its pure, solid form, this ring was made as a wedding band from solid, unalloyed rhodium. Paul Kiesow/Wikimedia Commons (CC By SA 4.0)

On Christmas Eve in 1800, English scientist William Hyde Wollaston and his colleague Smithson Tennant, unwrapped a gift — to themselves. It was a piece of nearly pure platinum ore, secretly purchased and smuggled from the Spanish colony of Nueva Granada in South America (Colombia today) for £795 ($1,051.99 in 1800 — worth $23,206.23 today).

The pair had high hopes for this single hunk of rock. Wollaston believed he could create a new chemical process that would make the solid ore a malleable platinum. Little did Wollaston know that his Christmas ore was the gift that would keep on giving. Their sample had secrets of its own hidden away — a new, rare metal never known to science before then is, today, the most valuable and precious metal on the planet, rhodium, a chemical element with the symbol Rh and atomic number 45.

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An Unexpected Discovery

With his chunk of smuggled platinum ore, in a few years Wollaston did what earlier scientists could not. He achieved a chemical process that isolated platinum and rendered it malleable.

As the scientist dissolved the platinum ore in his backyard garden laboratory, he produced both a soluble and non-soluble residue. After precipitating the soluble solution, he noticed reddish salts remained. Red salts are not typical of platinum, and Wollaston suspected something else was present in the sample. In 1803 and 1804, Wollaston announced that with the sample of platinum ore, he discovered two other precious metals. One he called palladium, and the other, rhodium.

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William Hyde Wollaston (1766-1828) was a pioneer of powder metallurgy who discovered rhodium and palladium. This selection of items includes Wollaston's slide rule of chemical equivalents and samples of platinum, palladium and rhodium with metal dies used to create wire.
SSPL/Getty Images

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What Is Rhodium?

Wollaston called the new metal rhodium, rooted in the Greek word for rose, "rhodon," because of the reddish salts that were dissolved in the aqua regia (aqua regia is a yellow-orange fuming liquid, so named by alchemists because it can dissolve the noble metals gold and platinum). "Rhodium is a part of the platinum metals group, which is considered one of the noble metals," says Shaun Peterson, Gemological Institute of America (GIA) supervisor of jewelry manufacturing arts research & development.

Platinum Group Metals, PGMs, include rhodium, platinum, palladium (discovered by Wollaston just a year before his discovery of rhodium), ruthenium, iridium and osmium. They have similar characteristics and are most often found together in nature. Like gold and silver, they are also precious metals. "Some of the key traits that precious metals share are allure, workability, durability and rarity," notes Peterson.

Rhodium is an ultra-shiny, corrosion resistant metal that had become useful in many industries including the automobile, jewelry, chemical and electrical trades. According to Peterson, it's rhodium's scarcity and use that makes it so valuable. "The rarity of rhodium and the large global demand due to the use in car manufacturing makes the price go up considerably," says Peterson. New regulations for cleaner emissions in the automobile industry, particularly in China and Europe, are most likely to blame for the increase in price.

Today, the price of rhodium is $14,000 per ounce. Compare that to platinum at $959/ounce, palladium at $1,866/ounce, or gold at $1,783/ounce.

When it is found, it's never found in its pure form. Rather, it's almost always collected as a minuscule byproduct of platinum, copper and nickel refining. If you want to go looking for rhodium, you better catch a plane to South Africa, the largest producer of rhodium by way of the country's massive platinum mining operations. You can also find it in river sands of North and South America, or in the copper-nickel ores in Ontario, Canada.

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Generally, Rhodium Is Not Harmful (Unless You're a Jeweler)

People have been mining rhodium as a by-product of platinum since the 1930s after large sediments of platinum ore were found in the Transvaal in South Africa. Since that time, there's never been concrete evidence of harm to humans, especially since it's highly unlikely for a human to come into direct contact with a substantial amount of rhodium.

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The chemical element rhodium after processing as a powder, as a pressed cylinder and as a remelted pellet.
Wikimedia Commons

Tests on plants indicate that rhodium is the least toxic member of PGMs, but because it's so rare, no tests have been done on humans. However, when breathed in, rhodium can be dangerous, Peterson says, particularly in jewelry making. "During the [rhodium] plating process, there are fumes that can be harmful. Due to this health risk, there are safety methods used when plating jewelry to avoid inhalation."

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Rhodium Keeps Our Air Cleaner and Jewelry Brighter

What do you, cars, jewelry and gum have in common? Turns out, a bit of rhodium.

Of available rhodium, 80 percent is used in catalytic converters in cars to clean exhaust emissions. Rhodium is uniquely exceptional at breaking down nitrous oxide molecules — aka, NOx emissions — the brownish, poisonous gas given off by fossil-fuel powered cars, trucks, boats, power plants and turbines, among many other offenders. Though the impact of NOx emissions cause irreparable damage to our bodies and ozone, it would be a lot worse without rhodium.

Rhodium is an essential catalyst for making nitric acid, acetic acid or hydrogen reactions. It's also a catalyst for making menthol, the minty flavor in chewing gum. Because it's resistant to corrosion and conducts electrical current easily, it's used as a coating for optic fibers and optical mirrors, headlight reflectors and electrical materials.

Humans are most likely to directly encounter rhodium in shiny, lustrous jewelry. "Jewelers seek rhodium to use in the jewelry making process because it is bright, silvery white in color, and very hard, which can help make the jewelry more scratch and corrosion resistant," says Peterson. "It is also hypoallergenic which can help those that may be allergic to certain jewelry metal alloys." It contains no trace of nickel, so even the most sensitive jewelry wearers can rest assured that their wrist is safe with a rhodium-plated bracelet.

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It's Not the Easiest Metal to Work With

Rhodium is extremely hard and has a very high melting point at 3,595 degrees F, or 2,035 degrees C, which makes it a difficult metal for jewelers to manipulate. "Rhodium by itself is too hard for general jewelry making purposes," says Peterson. "The most common use for rhodium is as a plating over other metal alloys, either to help protect against allergies, or to improve the color of the jewelry item." Its luster, shine and durability are definite pros to this rare metal, but because it's so hard to work with, its plating is thin and wears off quickly.

"The con," says Peterson, "is that the rhodium plating is only a thin layer coating jewelry. This means that over time the rhodium will reduce due to wear and tear." Jewelry makers prefer to use rhodium on pieces that experience less rubbing and wear, like earrings or necklaces and they recommend replating pieces every one to two years. Because you need so little rhodium to plate a piece of jewelry, it's relatively affordable. You can find rhodium plated jewelry online anywhere from $300-$5,000, but difference in price has more to do with the designer, base metal and inclusion of gemstones, more than the rhodium itself.

Thanks to Wollaston's discovery of rhodium more than 200 years ago, we can wear and drive our planet's most precious and pricey metal.

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