Though the game says that rhenium and lead are "very common" and groups them as Class-1 materials, they aren't common at all. You will find rhenium, lead and boron in any metallic or metal-rich ring, in roughly 1% amounts. So for every 100 materials fragments you mine, you'll average 1 each of these.
I mine enough that I have rhenium and lead to throw away, but I doubt that many other players could say that.
But since rhenium and lead are class-1 mats, it's probably far easier to trade a few class-4 mats.