Hey all, this is gonna be my first theory. I hope it hasn't been brought forward in this way.
I have been musing about the origins of Shardplate. I got this idea on my current reread of WoR, in one of Dalinars visions.
So i went back to WoK:
From the arrival of the KR, falling from the sky, it seems a good assumtion, that this is either a Windrunner or Skybreaker.
Same chapter, the KR that heals Dalinars vision character:
The next quotes are from Dalinar's vision at Feverstonekeep.
Dalinar looking out at the charging KR:
With these quote, it's a good assumtion, that the Orders had Plate glowing with specific colors.
What really got me into this theory, is the fact, that both Kaladin and Shallan attract huge amounts of Wind-/Creationspren.
From Kaladins first flight in WoR:
Syl was first described as a Windspren, though this is obviously wrong. But she shares some basic personality traits with them, i.e. playing pranks, flying around in highstorms. At multiple points in the story she calls the Stormfather her father, which seems like a good indication, that she is somewhat related to Windspren.
Now 2 quotes about Shallan, boath aboard the Winds Pleasure:
Here she is drawing Pattern, notice the amount of creationspren:
So with all this in mind, I went to check the WoB for any hints Brandon might have dropped there.
So with this quote I think it's save to assume, that Shardplate, much like Shardblades, were originally spren. Lesser spren, related to the bonded spren, that would form Shardplate.
A very likely spren coupling is Honor- and Windspren for Windrunners.
I presume something like this will come light for the other Orders as well. (Cryptics and Creationspren, maybe?)
TL;DR: Original Shardplate was formed by lesser spren.
edit: typos