Depends on the nature of the Historian and the distance in the future, if we're talking post western civilisation archaeologists plastics seem a good bet.
However if we're talking a couple of hundred years hence (when western civilisation still maintains hopefully) then I guess the "Information age" tag would hold and the associated material in the minds of arts professionals tends to be silicon.
I suppose it depends on what materials that we commonly use become less common in the future. Plastic seems obvious at first, but on further consideration I suspect that we've really only just *begun* to use plastic, and it will probably be rather a long time before plastics reach their peak. If so, the current era may end up with some other characterization.
Perhaps the more interesting question: which information formats will future historians associate with our era?