What I mean is there something like BrickColor.Random() for Material?
No, because Material values are Enums, and don't have functions that can be applied to them. On the other hand, BrickColor is a userdata value and has specific functions and properties.
Yes, there is a way, and here It is:
local Materials = {"DiamondPlate","Concrete","Brick"}--put all of the selections here game.Workspace.Part.Matieral = Materials[math.random(#Materials)]