An icon will appear next to the damage value when you deal damage to a mob. If there are small arrows above the icon, it means the mob is vulnerable to this type of damage and your damage will be increased. If there are two small arrows below the icon, it means the mob is resistant to this type of damage and your damage will be reduced.

Your overall damage is determined by your attribute points and weapon damage. Gems scale on individual attributes or weapon damage, whichever is higher. It's important to remember that some types of increased damage deal more damage than others. The table below also shows the best and worst damage types against each enemy category.