There was once a time where a pearl was a spectacularly rare thing. Cultured pearls have ruined that a little bit, but we haven't been able to replicate the trick that mollusks pull to produce nacre, the material responsible for both mother of pearl and pearls. Instead, we've just managed to improve on it, in a laboratory.
Nacre is the hard, iridescent stuff that oysters and some other mollusks use to line the insides to their shells to make it all smooth and comfy and pretty. It's also what pearls are made out of. Specifically, nacre consists of a composite of lots of different laminated layers of calcium carbonate, all stuck together with elastic biopolymers. What makes it iridescent is the fact that all those calcium carbonite layers are stuck together at nearly the same distance as the wavelengths of visible light, which causes different colors of light to be reflected at different viewing angles.
A paper published in Nature Biocommunications this week showcases a new method of producing artificial nacre using the same basic technique that mollusks do. Lots of layers of material are sandwiched together into a microstructure and glued together with polymers, and then soaked with a solution of calcium carbonate. The calcium carbonate slowly crystallizes, and you're left with a material with "nacre-like mechanical properties and striking optical iridescence" surpassing the real thing.
Rather than using this power for good (by selling fancy new fake pearls for tons of money and donating all proceeds to science research and education, say), the team from the University of Cambridge has instead decided to work on developing "tough coatings fabricated from cheap abundant materials." Oh well.