I found a stone plate that is named "Dragonstone", it has no value but wight 25 and i cant drop it...cant find out what to do with it?

It's a quest item.

Quest items thankfully have weight again but for some unfathomable reason are undropable.

So I suppose it is entirely possible to become overencumbered by undropable items.

That is so stupid, making quest item with weight but you can't store them????
Well tills i can make a mod for this i just add more strength to my character in the console, hope that still works.

Oh brother - i didn't realize you could not drop these things. I only have one, but as I'm just starting out, that 25 lbs matters. Need to find where to use it pronto.

its part of the main quest.