Pally holy damage is melee crit % based or spell ?

[Rinnhart]

As far as I know, Seal of Command is based off of your melee crit.  I'd assume that Hammer of Wrath is based off of your spell crit.
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I would say it's based off weapon crit chance. It deals double damage on a crit and can miss. We would, of course, need parses to prove either way.