The leading theory on Shardplate is that it is made of "cousin" Spren to the Radiant Spren. This idea has a lot of merit but one thing about it doesn't feel right to me, Shardplate functions as one object. If it were made of many separate Spren then it likely wouldn't be able to be regrown from any little chunk of the Plate. My theory is that Shardplate is the "Spren" of the Nahel bond. I think that when a Radiant reaches their fifth Oath their bond becomes a living entity, the cognitive representation of that bond. Radiants can then pull this Spren into the Physical realm in the form of Shardplate and can function as separate pieces because the Nahel bond has multiple facets to it. I also think that Radiant Plate is powered by the Radiant's innate Investiture and the Investiture of the Radiant Spren. So current Plate needs to be powered by Stormlight infused gems because it is the Spren of a broken bond. My evidence for this is that Plate grants physical abilities similar to breathing in Stormlight, its ability to be regrown from any piece and the linking with its owner that allows one to mentally command it to fall off.