The jewels nabbed in the Louvre heist may still be at large, but scientists have just closed the case on another gemstone mystery: what gives rare ammolite gems their rainbow shimmer.
Ammolite comes from the fossilized shells of extinct squidlike critters called ammonites. Scientists knew the secret to the fossils’ flamboyant appearance lay somewhere in their layers of nacre, or mother-of-pearl. But not all ammonite fossils boast brilliant colors — nor do pearly nautilus or pale abalone shells with similar nacre layers.
To find out why, scientists examined the stacked aragonite crystal plates that make up the nacre of ammolite, other ammonite fossils and shells of nautilus and abalone. Ammolite’s splendid colors arise from light reflecting off aragonite layers of uniform thickness, separated by gaps of just the right width, the team reports October 30 in Scientific Reports.
Materials scientist Hiroaki Imai became enchanted by ammolite at a mineral fair in Tokyo. “I thought it might have some kind of special coating,” recalls Imai, of Keio University in Yokohama, Japan. “I was astonished to learn it was the excavated fossil itself.”
Using electron microscopes, Imai and his colleagues inspected ammolite from the 75-million-year-old Bearpaw Formation in Alberta, Canada. They found that ammolite pieces with thinner aragonite plates reflected shorter wavelengths of light, creating deep blues, while thicker plates reflected longer wavelengths, creating rich reds.
The team could also see how ammolite’s microscopic structures differed from that of other, duller nacres. In ammolite, aragonite plates were separated by 4-nanometer-wide pockets of air. Proteins and other organic materials that once filled those gaps had been stripped away during fossilization. In abalone, 11-nanometer-thick layers of organic material still sat between the plates. And in a duller ammonite fossil from Madagascar, the plates had collapsed together.
Simulations revealed why 4-nanometer gaps were the sweet spot for rendering bright, distinct colors. More tightly packed plates didn’t reflect as much light, dulling their appearance. More widely spaced ones reflected a broad spread of wavelengths, muddling their color. Imai’s team also saw that the layers across a single piece of distinctly colored ammolite tended to have fairly uniform thickness.
Which ammonite fossils produce rich colors may depend on both species and preservation conditions, Imai says. His team’s next target: silica gemstones known as opals, which form through the weathering of rocks.
“Some types of opal exhibit vivid structural colors,” Imai says. “We are currently investigating whether the principles governing these vivid colors can be similarly explained.”