Matter converters are devices which can convert the mass of an object into another form, element, or set of molecules. The end result will have the same mass as the input. Because of this, lead is one of the best materials to use a matter converter on because it gives the most output compared to the volume of input and is the heaviest stable element/metal.
Limits
Matter converters do have limits on what can be obtained from them however, so there are many valuable materials such as gold that still need to be to be mined from asteroids or uninhabitable planets.