I think they are connected to radiants..I remember reading that knights rode Ryshadium.. although I am not sure m.. I think Dalinar mentions it somewhere.. I will try to find it..
ok found it.. Jenet says in the chapter “monster” in wor:
The one I found is Adolin in oathbringer saying that Ryshadium are called the third shard, blade, plate and mount..
I guess I took to mean that they are linked but it could be an Alethi thing too..