Elthan

I think Veil's answer is more likely than them being connected to spren. I believe they might have been created, by one of the Orders perhaps, to fight the Voidbringers like Adolin mentions after being hit by the red lightning. Even if this isn't true and this is just some way the armor adapted (he did get blinded a few times before this) I think that there's definitely something about how the armor is powered by Stormlight and might not have used gemstones before. Just think about how Kaladin used the helm when he fought in the Arena. It repaired by using his Stormlight. This theory does not lend itself very well with how they could summon and unsummon it at will, but I still think that that could be possible if you think about the Oathgates. It's a fabrial (correct me if I'm wrong here) of some sort that uses Transportation, maybe the Plates are something similar to that.