Watchstones are quest items for the Atlas of Worlds system. There are 32 Watchstones in total. It was introduced in Conquerors of the Atlas expansion.
The Atlas is divided into 8 regions. Each region of the Atlas has 4 Watchstones in different colours, corresponding to 4 Conquerors of the Atlas. Players are required to beat the bosses in their citadels of that region in order to gain the Watchstones. However, if the Watchstone of that region of that boss was acquired already, beating the same boss again will not give the second copy of the Watchstone of that region. Additionally, the Final Conqueror, Sirus, Awakener of Worlds, drops unique Ivory Watchstone.
If you want to perform better in the path of exile, you can buy Path of Exile Currency at Eznpc, which will make your game road more smooth.