Anita Radini, an archaeologist at the University of York , in England, spends a lot of time looking at tartar. Really old tartar.

Tartar, or dental plaque — that film of bacteria that feels like sweaters on your teeth — contains a wealth of information about what long-dead individuals encountered in their daily lives. Dr. Radini has seen all sorts of things trapped in it: food particles, textile fibers, DNA, pollen, bacteria and even wings of tiny insects.

But several years ago, when studying the dental plaque of a nun from medieval Germany, Dr. Radini saw something entirely new: particles of a brilliant blue. She showed the findings to Christina Warinner, another tartar expert, who was shocked.

“They looked like little robins’ eggs, they were so bright,” said Dr. Warinner, group leader of archaeogenetics at the Max Planck Institute for the Science of Human History in Germany. “I remember being dumbfounded.”