I recently made a post with roughly the same meat and potatoes, so to speak, on Wowhead, and I thought I'd share some of my insights here with you all, for all of you Protection Paladins who are a bit confused about meta gems at the moment.
Let me first start off with the TL;DR version.
The only meta gem you - as a PvE Protection Paladin, in a raid-ready environment - should be using is the Eternal Shadowspirit Diamond.
If you're looking for a quick answer, that my friends, is it.
If you want to know why, please read on.
Now, I'm sure many of you have heard that this item is 'bugged'. It says 5% shield block value, but only yields a net gain of 1% shield block value.
This is, according to several people on Wowhead who claim to have contacted a GM, 'working as intended'.
This is not because it is multiplicative - else, when we have Holy Shield up, we should have 42% shield block value. (We don't.)
Instead, it is allegedly because of a bug in the tooltip of the item itself.
Now, I'm not taking the word of folks on Wowhead as being etched in stone. It could very well be that this gem will one day provide 5% additional shield block value. I sure hope it does. That'd make it even better!
However, I went through a few calculations, to see if the (possibly bugged) 1% shield block value offered by the Eternal Shadowspirit Diamond was better or worse than 2% armor from items, as offered by the Austere Shadowspirit Diamond.
Not surprisingly, it was.
It's important to remember that armor's damage reduction is dependent upon the attacker's level.
The tooltip from armor on your character pane indicates the damage reduction from level 85 sources, and not level 88 (boss level) sources.
The equation for damage reduction from armor is as follows:
DR% = 100 * Armor / ( Armor + 2167.5 * Level - 158167.5 )
In other words, for level 85, it is:
Armor / (Armor + 26070)
And for level 88 (boss level) mobs, it is:
Armor / (Armor + 32572.5)
With 2% additional armor - and bear in mind, the 2% additional armor from items is based on the items, not any talent you may have - the increase in damage reduction is negligible.
Currently, we're looking at about 32,000 armor from gear, give or take, for full ilvl 346 geared Protection Paladins.
Please don't respond to this with something stupid like, "You're wrong Atrea, I have 40k armor in my character pane, in ilvl 346 gear", because we aren't counting armor from talents or abilities; simply armor value from items.
And yes, you may have slightly more, depending on your trinkets. You might also have slightly less.
32,000 is the ballpark figure, though. I currently have ~31.7k in full ilvl 346 or better, with no armor trinkets, but several ilvl 359 pieces. So it's even a little high.
As such, with 32,000 armor, your gain from 2% armor from items is simple; 640.
Now, let's assume, with all your talents, armor buffs (Devotion Aura or Stoneskin, whatever you like), you can reach a total of 40000. (Yes, I realize you can reach a bit higher if you truly cared to. However, let's just pick 40000 for the hell of it.)
According to the formula I listed above, against a level 85 target, 40k armor reduces incoming physical damage by 60.54%
However, against a level 88 target, it only reduces it by 55.12%
Now, let's apply the Austere Shadowspirit Diamond.
You gain 640 armor. (Remember, 2% armor value from items, not from your total buffed value!)
Against a level 85 target, 40640 armor reduces incoming physical damage by 60.92%. A gain of .38% damage reduction.
Against a level 88 target, it only reduces it by 55.51%. This is a difference of .39%
Ultimately, in order for Austere Shadowspirit Diamond to be more worthwhile than Eternal Shadowspirit Diamond, gear would have to improve to the point that you gained enough armor from items (before talent modifiers and abilities) to exceed a damage reduction bonus from the gem significantly higher than your shield block chance, divided by 100.
How does this work?
With Eternal Shadowspirit Diamond, you gain a flat 1% block value increase. Your damage reduction is therefore your shield block chance x 1%, or divided by 100. To put it into practice, if you block 1% 40% of the time, you are reducing damage by .40%
Ergo, SBC x 0.01 = DR from 1% SBV.
Right now, against a level 88 target, a flat .39% damage reduction is easily eclipsed by a 1% increase in shield block value. Mastery is abundant enough on gear, and can be reforged, to vastly increase your shield block chance beyond 39%.
Additionally, the requirement of Austere Shadowspirit Diamond, two yellow gems, while not insanely awful, does cause a little strife, in that yellow (or hybrid yellow) gems are typically not socketed unless in yellow sockets, and even then, only if the socket bonus is worthwhile - the definition of which varies from player to player. The point is, however, if you switch out gear that has the yellow socket for one with a blue or red, you may have to re-gem your gear again, or even skip socket bonuses - which, with anything other than a stamina gem, is an awful decision to make.
To summarize, until later tiers produce enough armor on items to grant more damage reduction from a 2% increase to their value than 1% of your block chance, Eternal Shadowspirit Diamond is the meta gem of choice if you are a Protection Paladin.
Furthermore, while I don't have a Protection Warrior - my Warrior is PvP Arms/PvE Fury - I am reasonably sure that everything I said here applies for them as well.
PS. This wasn't an incorrectly-posted 'guide contest' entry.