Silver

“Old Neville here woke me in the Martian wastes. Utterly alone, with nothing but our wits— and a lone, silvered coin buried in the sand beside me. This very coin, in fact. What could it signify? A mysterious message? A sign of royal birthright? A key to some ancient puzzle? For all my many adventures, this answer yet eludes me— What's that?”
- Fenchurch Everis
Long before the Collapse, the Reef settlements used a currency commonly known as Silver— coins with engram-like qualities which could be digitally signed with an individual person's key.

Tess Everis, born in the City after her parents fled the Reef, counts among her most prized possessions an old Silver coin that belonged to her Reefborn grandmother. As the Silver was cryptographically unique, she was stunned to meet an Awoken Warlock named Fenchurch who possessed around his neck a Silver coin of his own, signed with the exact same key as Tess's. Like all Guardians, Fenchurch has no memories before the first time his Ghost resurrected him. But their Silver coins' shared origin leads Tess and Fenchurch to suspect that they are related.

Fenchurch instantly took to thinking of Tess as a long-lost niece, even assuming her surname, Everis. Tess loves the Silver that her new partnership with Fenchurch brings— but, though she'd never admit it, she secretly values her newfound family even more.