Ood

Better yet:

Give it 5 gem slot spaces

Make any gem socketable to belts.

And give it invulnerability proc while you're at it, who needs damage?

Agree completely with OP.

A friend recently asked me what gem I would recommend him putting in his helmet (currently has tourmaline). He was considering HP/MP/MP regen.

Based on the maths, HP/MP are not worth anything in my book.  

As a DPS, in a proper party with a good tank, you won't be taking any damage. Should you do so, +500 HP (glorious) would likely let you survive an extra hit or two depending on your defense gems. Sure, that may be helpful, but placing a defense gem in slot would likely help you survive longer, if damage types match up (%s are in disagreement as posted in OP). On the other hand, +500 MP will let you cast an additional 2 to 3 skills. Again, this is a pretty minor advantage for such an expensive gem. HP regen has been shown to be absurdly small (Wherever irradiated made that post). Finally MP regen does seem to have a noticable effect (to me), but the maths behind it still show it to be very minor even at high tierms.

In the end, the recommendation to the friend, was:

Do nothing, save your 50 EP unslot fee, sell all HP/MP gems, buy attack power gems.

Also being able to equip hp/mp gem into weaponry is similarly absurd.

A Glorious almadine nets you 138 HP per hit when socketed to a weapon. 500 HP  for glorious jasper is nothing in comparison.

Belt:

Instead of gem slots, give them increased potion slots, perhaps do this by item grade instead of tier:

Normal: 2 pots

Fine: 3 pots

Normal crafted: 3 pots

Special crafted (add some if there aren't any): 4 pots

Rare: 4 pots

Heroic: 5 pots

Fabled (add some): 6 pots

(or whatever the item tiers are, those listed are just example)

@ Jewelry gems

One of two things:

What you suggested (1 slot each for rare crafted jewelry), limit gems that can go into jewelry to: Aqua, tourm, Jasper, turq, howl (perhaps even make these only able to go in jewelry [except perhaps howl]).  This would be fine, though a bit boring.

Something more fun might be:

Add a fifth jewelry slot, independent from the others (don't make 5 item group sets). Make it an 'heirloom' type item that grows with your character. Obtain it in a low level quest, then 'level' it as you level/get exp for your character.  Allow it to change properties based on your gameplay (heal more, it eventually asks you if you want to move it towards healing properties; warrior more, it eventually asks you if you want to move it towards more 2h sword specialities, etc.), and/or allow it to 'consume' equipment to change properties (consume multiple light armors to give more sorcery, consume vitality heavy jewelrys to give vit bonuses, consume large amounts of mundanes to increase total effects, etc). Obviously limit these bonuses somehow, but make it another activity to pursue at the end game, in addition to gem crafting.

Rake answered this above. The order in which you consume food/pots etc. affects the order in which they are applied and therefore the overall dps. Log out and back in and the order changes, presumably to an inbuilt default.

Ah, I see:

Expanding on this formula above:

([Base DPS * DPS factors] * skillfactor)*1.05 = damage

@ Dmg obelisk

It adds 5% 'Damage'

Damage ≠ DPS

The ob adds 5% at very end to damage count, it isn't a boost to attack power or DPS directly. So your damage formula with damage obelisk is generically:

([Base DPS * DPS factors] * skillfactor)*1.05 = damage

Your ranged pot, food, and attack pot go under 'DPS factors', and the 5% damage is tacked on the end of whatever you are casting at the time.

Why bleeds are disregarded, I have no idea.

Why you get a different boost when re-logging is also strange, the effects should be cumulative.