Gem blast is basically like a more potent version of the Keeper's staff of the Cosmos. Same proc and AOE. Just no crit. I will be releasing a big guide on this at some point. Like the Keeper's staff, you can do additional damage by casting frostbite onto a target and freezing it (in fact, possibly more than the Gemstone).
Gemstone by contrast does slightly higher damage to a single target, slightly better armor, but no AOE.
Both staffs give the same skill damage bonuses. Both staffs will trigger the set bonus as well. I have a PDF comparing the 3 mega mage sets to the shadow sets that I will be releasing. On my to do list are comparisons to the cosmos set and other level 55 sets. Also, please disregard the calculations that the OP has made - as Royce has noted, they are not accurate and not reflective of real game conditions.
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