Warning: There be theorycraft and numbers involved in this post
I've just recently upgrade Keluin's helm to the non-set EoT one - Heaume of the Restless Watch and a post by the new Disc/Holy priest columnist over at wow.com got me thinking about my meta gem choices. What a paladin reading a priest column... especially when his highest priest is level 22? Actually the title sucked me in because the holy paladin meta of choice is Insightful Earthsiege Diamond.
Summary of the post is this: although the on chance effect of the meta looks attractive it only works out as about 60-75 mp5 and at current gear levels 2% Intellect from Ember Skyflare Diamond is likely to be worth more than the static +21 int (I'm ignoring Beaming Earthsiege Diamond because int provides more than just mana for paladins).
On the surface value you only need
intellect to already see the benefits of the Skyflare meta (the 10% multiplier is because of Divine Intellect). My paladin currently has 1342 intellect unbuffed - looking at this at surface value I would therefore see more benefit from using the skyflare diamond gaining about 26 int unbuffed.
21*10%/2% = 1155
However, what isn't factored into this calculation is the activation requirements. The Earthsiege can be activated with one gem - a Nightmare Tear leaving the other two gems be the standard +20 int gems. The Skyflare however requires 3 red gems, which means that you will have to use alternative gems in other locations just to meet the meta. At best this means that you will only be able to gain 30 int total from those gems when using a Luminous Ametrine. So if you add those numbers together the value of the Insightful Earthsiege Diamond is:
So that is a base requirement of 2050 int before you'll see the benefit of the Skyflare diamond purely in terms of int gained.
21 + 10 + 20 + 20 = (int*2%) + 10 + 10 + 10
int = 41/2%
int = 2050*10% benefits of Divine Intellect
int = 2255
The other benefit of the Skyflare is the gain of spell power - 61 in total assuming you use three Luminous Ametrines. Again looking at my paladin's stats I have 2325 spell power all from gear - so this is just a 3% increase in my spell power. (Note I haven't taken into account the spell power gains from Holy Guidance but the difference is going to be somewhere between 1 and 2 spell power).
Finally, the one thing I haven't calculated in this is the difference in mp5. Combining Divine Plea, Replenishment and Arcane Torrent usage results in 65.5 extra Mp5 per 100 Intellect (I got that figure from this post over at HolyPaladin.net). The 60 - 75 mp5 from the on chance use therefore would require your int to be in the range of 5000 int to see equivalent mp5 (and be a blood elf...). The difference here is that mp5/mana is a throughput stat for Holy paladins - therefore the value is somewhat higher than that of priests (have a look at this thread on the wowhead forums for an interesting discussion of disc priests versus holy paladins and mana/spellpower requirements... if you just wade through all the flaming).
My conclusion: stick with the Insightful Earthsiege Diamond for now unless you are gearing for spell power and going with a Flash of Light focus then it might be worth it (I'm not sure what the base stats of the FoL gearset would be like - I currently have 180 int coming from gems so I would still be above the initial 1155 int mark). Once your unbuffed int is somewhere is the range of 2255 then it might be worth looking at the benefits of more spellpower instead.