@Julli-of-North I can quickly answer this, as I had a similar question.
You can choose between Gems or Cursed Runes as long as you don’t have the required amount of Cursed Runes for the upgrade (or more than enough).
Once you have enough Curse Runes, it will only offer the upgrade using Cursed Runes only.
But I have passed this on to the team!
As Cursed Runes are new, I understand the value of wanting to save them for recipes and the option to choose between Gems or Cursed Runes would be beneficial even if someone has enough Cursed Runes for the Soulforge upgrade.
I’ll update the Game Guide article on this in the meantime to make this clearer.
Jeto - Support Human 