Byzantine Coins are found in multiple locations in Rise of the Tomb Raider, where they work as currency.
Byzantine Coins can be found e.g. in the form of stacks of Coins, in Coin Caches, or inside Gift of Alms. Lara can collect these and later spend them at the Supply Shack in the Soviet Installation to buy certain Gear upgrades. They are pure gold, rather valuable, and untraceable, the latter being the reason that the renegade Trinity Equipment Technician who runs the Supply Shack prefers them over normal money.