It was aluminum; aluminum is the only Investiture blocking substance we know of.
Apparently it’s not that easy to doulcast aluminum, as it ends up being a major Scadrian export. If it was that simple you’d expect it to be a Rosharan one, but it isn’t.
Considering Lin Davar’s Ghostblood connections, there’s also the possibility that the necklace did not originate on Roshar.