The warlock conjured Spellstone items all say they increase damage dealt by periodic spells by 1%, but they are not increasing the damage of Curse of Agony or Curse of Doom. These are the only periodic spells that don’t have their damage increased by Spellstones. Every other source of periodic damage increase (the talents Shadow Embrace and Haunt) increase the damage of Curse of Agony and Curse of Doom.