Ytterbium is a soft, silvery rare-earth metal with atomic number 70, best known today for its role in high-power fiber lasers, though its path to discovery was a tangled, decades-long story involving mistaken identities and scientific rivalry.

Its name comes from Ytterby, a tiny village in Sweden where a mineral containing the element was first found. The village is legendary in chemistry for lending its name to four different elements: yttrium, terbium, erbium, and ytterbium.

The Swiss chemist Jean Charles Galissard de Marignac discovered it in 1878 while analyzing a rare-earth sample called erbia, though the Austrian chemist Carl Auer von Welsbach independently found it around the same time and named it “aldebaranium,” a name that ultimately lost out to ytterbium.

What Marignac had actually found wasn’t a single pure element. In 1907, the French chemist Georges Urbain split it into two separate elements, ytterbium and lutetium. This helped resolve a problem that had kept scientists disputing ytterbium’s atomic weight for decades, as they hadn’t realized they were working with a mixture.

Even after its 19th-century discovery, a truly pure sample of the metal wasn’t obtained until 1953.

The element is relatively rare, making up only about 3 parts per million of the Earth’s crust. Most of the world’s supply is mined and processed in southern China from ion-adsorption clays, with global production of ytterbium oxide remaining fairly small, estimated at only 50 to 100 tonnes a year.

It’s a soft, malleable, ductile metal with a bright silvery luster that slowly tarnishes in air, reacts with water to release hydrogen gas, and dissolves readily in acids.

It has three distinct crystal structures that shift at different temperatures, and it has one of the smallest liquid temperature ranges of any metal, with a melting point of 819 degrees Celsius and a boiling point of just 1,196 degrees Celsius.

While its most stable oxidation state is +3, it’s unusual among rare earths for also having a relatively stable +2 state, which makes it a strong reducing agent. Solutions in the +2 state have a distinctive green-yellow color, while those in the +3 state are colorless.

Under high pressure, around 16,000 atmospheres, ordinarily metallic ytterbium even turns into a semiconductor. Because its electrical resistance changes so dramatically under pressure, it’s used in sensors that detect underground explosions and earthquakes.

Ytterbium’s most important modern application is as a dopant in fiber lasers used for industrial cutting and welding. Most high-power industrial metal-cutting lasers today are ytterbium-doped fiber lasers, valued for being more efficient and having better beam quality than older laser types.

The specific wavelength these lasers produce is well absorbed by water and blood, which makes them useful in varicose vein treatments and soft tissue surgery. Ytterbium fiber lasers are also central to directed-energy weapon concepts, including systems tested by the U.S. Navy.

Beyond lasers, ytterbium oxide emits more infrared light than magnesium oxide, making it a key ingredient in decoy flares designed to confuse heat-seeking missiles. Meanwhile, a radioactive isotope, ytterbium-169, is used in portable X-ray machines for situations without access to electricity.

Small amounts of the element added to stainless steel can improve its strength and refine its grain structure, and ytterbium fluoride has even been researched as a nontoxic dental filling material that slowly releases beneficial fluoride ions.

Some of ytterbium’s more remarkable applications lie at the cutting edge of science.

Ytterbium ion clocks rank among the most precise timekeeping devices ever built, accurate enough that they wouldn’t gain or lose a single second in 30 billion years, longer than the age of the universe itself. Researchers are also studying the element as a dopant in semiconductors for potential use in quantum information technologies and quantum bits.

It has no known biological role in the human body and is considered to have low toxicity.

Its correct chemical symbol is Yb, though some early or less accurate sources have mistakenly listed it as “Yt.”