This is super geeky, but Ric first asked about the Dothraki and his follow up mentions the Unsullied. The Unsullied have dragon glass spear points to my eye (yeah, yeah, it's dark). The Dothraki do a lot of head lopping and that seems to slow down the dead so maybe that was the original plan.
Who fucking knows. This is all TV writers and little to none of the author so there isn't going to be as much clarity or consistency.