So by that logic, could the half shard shields that they talk about in Oathbringer be made of Aluminum? That might explain why they are able to withstand a few hits from a Shardblade. Or are the half shards just trapped spren in fabrials that can change their shape like radiant spren can, but since they are in a fabrial and not actually bound to the physical realm or a person because there is no oath holding them in form?