Assuming that you chose to place skull gems in all of your jewelry sockets, that would give you at least 750 armor. However, if you have a few pieces of gear with +%armor on them, skull gems may actually give you as much as 825-850 armor (assuming +10-15% armor).
I’ve looked through the descriptions for the different malignant hearts, and the only one that seems to give constant damage reduction is Revenge. Revenge states that 10-20% of incoming damage is instead suppressed before exploding to damage nearby enemies.
If damage suppression = damage reduction, then Revenge is probably worth the 275 armor you would lose from a single skull gem. That still leaves 2 gem slots or 550 armor (assuming +10% armor from gear). Do the damage increases or utility from the other hearts make up for losing 550 armor? Given the nerfs to survivability with the latest patch, I’m not convinced it does. What do you think?