Well... forgive me if I am incorrect, but I believe that many materials harnessed by humans are tainted in some way. They are mixed together or hold imperfections that could harm the spirit. In the worst cases, they may be cursed.

I believe that some form of clear crystal or gemstone could provide a suitable vessel provided it has no imperfections. Though if no such pure material exists on its own, I am sure there are others here who could purify it for you.