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Druids are only capable of dodging attacks, they rely on their dodge, high armor, and high hitpoints to make up in areas where other tanks excel. They make excellent tanks against caster bosses due to their high hitpoints and do fairly well in heavy melee fights although they lack a lot of the mitigation of some of the other tanks. However, Druid Tanks work very well to tank just about all content, raids and instances, while undergeared, they will struggle against raid bosses in the end game if they remain undergeared but are really solid tanks.
So the gems you are going to focus on will be a combination of stamina, agility, and dodge.
Some gems that you will get away with if there's really no other choice are defense. And of course you'll have your threat generation gems, hit and expertise, which you should only use if you really suck..I mean if your gear is still lacking it.
Gems you will want to avoid for tanking are strength and parry, these help other tank classes but not Druids.
Epic Tanking Gems for Druid by Slot Color
These are the epic tanking gems for Wrath of the Lich King, if you are cheap or still leveling, I don't recommend you buy these unless you're rich.
Red
Shifting Dreadstone (+10 Agility, +15 Stamina)
Regal Dreadstone (+10 Dodge, +15 Stamina)
Delicate Cardinal Ruby (+20 Agility)
Threat Generation
Precise Cardinal Ruby (+20 Expertise)
Guardian's Dreadstone (+10 Expertise, +15 Stamina)
Glinting Ametrine (+10 Hit, +10 Agility)
Yellow
Enduring Eye of Zul (+10 Defense, +15 Stamina)
Stalwart Ametrine (+10 Dodge, +10 Defense)
Thick King's Amber (+20 Defense)
Threat Generation
Vivid Eye of Zul (+10 Hit, +15 Stamina)
Resolute Ametrine (+10 Expertise, +10 Defense)
Glinting Ametrine (+10 Hit, +10 Agility)
Blue
Solid Majestic Zircon (+30 Stamina)
Shifting Dreadstone (+10 Agility, +15 Stamina)
Regal Dreadstone (+10 Dodge, +15 Stamina)
Threat Generation
Guardian's Dreadstone (+10 Expertise, +15 Stamina)
Vivid Eye of Zul (+10 Hit, +15 Stamina)
Meta
Austere Earthsiege Diamond (+32 Stamina, +2% Increased Armor Value)
Effulgent Skyflare Diamond (+32 Stamina, -2% Spell Damage Taken)
Blue Tanking Gems for Druid By Slot Color
Red
Shifting Twilight Opal (+8 Agility, +12 Stamina)
Regal Twilight Opal (+8 Dodge, +12 Stamina)
Delicate Scarlet Ruby (+16 Agility)
Threat Generation
Guardian's Twilight Opal (+8 Expertise, +12 Stamina)
Resolute Monarch Topaz (+8 Expertise, +8 Defense)
Glinting Monarch Topaz (+8 Agility, +8 Hit)
Yellow
Enduring Forest Emerald (+8 Defense, +12 Stamina)
Stalwart Monarch Topaz (+8 Defense, +8 Dodge)
Thick Autumn's Glow (+16 Defense)
Threat Generation
Vivid Forest Emerald (+8 Hit, +12 Stamina)
Glinting Monarch Topaz (+8 Agility, +8 Hit)
Rigid Autumn's Glow (+16 Hit)
Blue
Solid Sky Sapphire (+24 Stamina)
Shifting Twilight Opal (+8 Agility, +12 Stamina)
Regal Twilight Opal (+8 Dodge, +12 Stamina)
Threat Generation
Guardian's Twilight Opal (+8 Expertise, +12 Stamina)
Vivid Forest Emerald (+8 Hit, +12 Stamina)
