Is there anything that you DON’T get a kick out of?? ;HilariousPumice does not contail any metals, per se. It may contain metal oxide(s). Also, there are applications where pumice is coated with metal oxides to increase its capacity.
For purposes of using pumice in aquariums, it is inert.
To make the water murkier (pun intended), no one knows what Matrix is composed of other than the people at Seachem. And to add some mystery, Seachem does not provide specific answers to questions about the properties of Matrix. Furthermore, they will neither confirm not deny the presence of metal oxides in Matrix.
As @Magellan alluded, the presence of GAC will remove most metals through adsorbtion. Side note; I get a little bit of a kick out of it when people/companies say that GAC absorbs anything. GAC does not absorb anything but does adsorb all kinds of different properties.
but great explanation!