The information from the
Relentless Earthstorm Diamond – melee only? thread (much of it thanks to Roywyn, along with the damage testers) might be of some use. Here's a sum-up for:
100% +
{(150%*1.03) - 100%} *
(100% + TDI) = Crit Damage
Where
100% is the normal damage
{(150%*1.03) - 100%} is the crit damage with the relentless earthstorm diamond (would normally be 150% - 100% or simply 50%). This would be {(200% * 1.03) - 100%) for melee damage.
(100% + TDI) is the total damage increased from talents, with TDI as the percent increase from talents (100% for ele shammies using Elemental Fury, Locks using Ruin, Moonkin using Vengeance; 50% for arcane mages using spell power; 30% for rogues using lethality; etc)
For simplicity's sake, here's a list of crit damage increase from the Relentless Earthstorm Diamond:
(Note that Ignite is not a crit modifier – it just multiplies the total damage done by 1.4. Ignite is also included in the final crit damage %)
Moonkin: 209% - 200% = 9%
Feral Druid: 216.6% - 210% = 6.6%
Marksmanship Hunter: 237.8% - 230% = 7.8%
Arcane Mage: 181.75% - 175% = 6.75%
Arcane/Frost Mage: 236.25% -225% = 11.25%
Fire Mage: 216.3% - 210% = 6.3%
Arcane/Fire mage: 254.45% - 245% = 9.45%
Lethality Rogue: 237.8% - 230% = 7.8%
Elemental Shaman: 209% - 200% = 9%
Ruin Warlock: 209% - 200% = 9%
Arms Warrior: 220% - 227.2% = 7.2%
As you can see, Arcane/Frost mages get the most benefit from this gem, whereas pure Fire mages get the least out of it. It is, however, at least double, if not triple, as powerful as the tooltip would lead you to believe.