Opals are a type of currency that you can use in Assassin’s Creed Valhalla and it’s spent in markets.
Markets will be unlocked later in the game within settlements. You can get opals from exploration or from contracts.
Here’s a map of all the opal locations in both Rygjafylke and Hordafylke (Norway) in AC Valhalla.
Opal locations in Hordafylke
Below is a map of all the opal locations in Hordafylke. If you wish to have all of these shown in your map, you need to spend some real money in the Ubisoft Store.
Opal locations in Rygjafylke
Below are all of the opal locations in Rygjafylke. Hopefully this helps.