You create it from silver and gold ore, but only after you find the recipe note in the main questline.
Viewing post in Legend of the Runekeeper...v3.4 comments
Oh! Ok!
My mistake.
Originally, it was a part of the main questline, but I change it.
Go to Rosengrad and speak to the vampire guy (at night) there after doing his first side quest. He will tell you about a problem in the local coliseum.
Head to the coliseum and fight your way down into its depths and you will fight an enemy called the 'Amalgamate'.
It will dropped the item 'Ruined Research'.
This is how you create platinum.