5e rules says a fireball is pure flame that sets light to flammable objects that are not carried or worn. Most rocks don't burn ( oil-bearing shale does ); most metals don't burn.
You COULD argue that, in fact, almost anything will burn if you pour enough energy into it, but that argument is not implied by the 5e description.
The Larian version of fireball looks like it coats everything in viscous burning goo; probably because they are simply reusing the mechanics of exploding barrels of viscous burning goo.
That could just be laziness; or it could be that for unexplained story reasons, the entire FR world around Baldur's Gate is somehow coated with viscous goo waiting to be set alight.
