Mind cores are the corrupted cores of Mind Golems, imbued with the power of the Mind Altar. They can be used in place of rune or pure essence, and acts as if the player was carrying ten essence instead of one, meaning each core gives 55 Runecraft experience and ten mind runes.
Like regular essence, the amount of mind runes crafted will increase with the player's Runecraft level; for example, each mind core will craft 50 mind runes at level 56 Runecraft.
Golem cores cannot be placed in a runecrafting pouch. If players attempt to store them in one, a chatbox message will appear, stating "The golem core is too big to fit in an essence pouch."
For an exhaustive list of all known sources for this item, see here