Work in progress ……… with some spoilers
O.k. found 3 pieces so far all found during the side quest "Kogog'Aak" in the broken spear pass when you opened the puzzle door you will then proceed through the horrid maze of a place whilst exploring a progressing through be thorough and you should find them easy enough in chests ….the last piece I got was after beating (spoiler...…) the Sun Queen at the end of this dungeon to get to her you need to open the final door it takes four sun stone pieces to open the door the locations of them are as follows :
1 the candle puzzle in the chest found whilst doing the "lost cargo-The Reaper quest"
2 the border camp solve the totem puzzle in the camp and you will get the 2nd piece
3 in Dura'Tyr there is a house SW of the entrance to the Ziggurat of the Sun the two doors close together in one of this rooms is the 3rd piece
4 last bit of the stone is in the Ziggurat itself in the room with the mirrors there is an rather obvious sun indent in the north east part of the room right by the cenotaph..shine a mirror on it switch do devourer and collect .
now back to the sun quuens chamber ..kick her floating arse (be careful shes lvl 16 and seemed to be able to one shot me randomly once!) then she drops the breast plate
that's all I have managed to find so far if anyone else finds any pieces please post and we will add it with full credit to the finder ..hope this helps guys