Mimic stones are for styles and jewelry doesn't have styles. It would make zero sense to have mimic stones mimic refined materials, but only for jewel crafting. That would add unnecessary confusion.
However, in order to save you some effort finding the appropriate level material, I suggest they add a new item to the store that will function the way you want it to for refined materials. I'll call it MimicMat. Each MimicMat will replace 1 refined material such as platinum ounces or even rubedite ingot. The cost per MimicMat will be 500 crowns each. You're welcome.