Pearl

John Who found a pearl inside an oyster. (COMIC: Guests of King Neptune)

A pearl was a type of gem that grew inside an oyster.

John found a pearl in the oyster he was eating at King Neptune's castle, while Gillian was given a pearl necklace. (COMIC: Guests of King Neptune)

A Silurian of advanced age would festoon the third eye with pearls as a form of decoration. (PROSE: Blood Heat)

Fitz Kreiner described the planet Yquatine as a "blue-green pearl". (PROSE: The Fall of Yquatine)

Lady Babbington's pearls were stolen by Ada Mullins, a jewel thief better known as "the Unicorn", in December 1926. (TV: The Unicorn and the Wasp)