Gold coin ('hyperpyron'), Constantinople (now Istanbul), Byzantine Empire, 1081-1118 AD

TIL I learned that Byzantine ‘mints’ would use one concave and one convex die to intentionally produce slightly cup-like, wavy coins. It seems this was done both to strengthen thin coins, and also made them harder to reproduce.

Couple sources here:
https://coinweek.com/featured-news/byzantine-coins-become-cup-shaped/
https://eprints.whiterose.ac.uk/124522/8/fuegrev2.pdf