Melee physical damage / Weapon elemental damage / Added fire damage

Which gem can give the best boost for glacial hammer?
And could you explain it further for me?

Glacial hammer converts a majority of your physical damage to elemental, so weapon elemental damage with its multiplative 70% increase is the way to go.

I THINK increased melee physical increases the base amount of physical that is then converted to elemental. I may be wrong on this. It's worth using with any melee skill.

Added fire is just as strong as melee physical however it is a different element than the main skills damage. You'll be more likely to get long chills and freezes with a high amount of one element being dealt. However a crit will lead to burning, however useful that may be to you.