Heart of the Golem
May cause unexplained addiction to flowers.
It can be found in chests generated within Simple Dungeon, Abandoned Mineshaft, Village Armorer House, Stronghold, Desert Pyramids and Jungle Temples.
While equipped in Spellstone slot, it will give the bearer +100% Knockback Resistance, +25% Melee Damage Resistance, +40% Explosion Damage Resistance, immunity to pressure and pricking damage, vulnerability to Magic Damage, and +4 Armor Points. The last bonus is substituted with +16 Armor Points and +4 Armor Toughness if they wear no armor.
The nature of spellstone's effects make it a perfect substitute to any armor on early stages on the game, because of powerful protection and, unlike with most of the regular armor - no durability loss. The obvious downside is that it doesn't allow the player to benefit from enchantments like Protection if they decide to wear no armor, so eventually this feature should give way to enchanted diamond or even iron armor as you progress.
But even then, Heart of the Golem remains one of the most powerful and universal spellstones available to player, adding a lot to their sustain... Unless they are fighting Ender Dragon or something.
Almost any direct damage is considered to be Melee Damage.