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Death Knights mitigation abilities include dodge and parry. LIke the Druid they are not able to wear a shield so Block and Block rating will serve no purpose. This also means that stacking strength will not help your mitigation although it will help threat by increasing attack power. Your main tanking stats then are doing to be Defense, Parry, Dodge, Agility, Armor, and Stamina.
Death Knights are typically high in Parry and armor by default. This is party due to the fact you get weapon enchants early on that add parry by the percent to your weapons which works better than the defensive enchants you get until you get a lot of stamina. Unlike the other tank classes though, Death Knights can dual wield tank, the only downside is without a lot of expertise you will take extra damage from a boss due to hastened parry effects if a Boss were to parry you since he now has double the chance, other than that, it works great and is fun and unique.
Dual Wield tanks will want to stack expertise for their threat generation if possible, it is the most important threat generation stat until you get about 6% because a boss can parry or dodge your attack and it removes chances for both at the same time, whereas hit only reduces miss.
Gems you will want to avoid for tanking are strength and anything with block on it.
Epic Tanking Gems for Death Knight 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
Regal Dreadstone (+10 Dodge, +15 Stamina)
Defender's Dreadstone (+10 Parry, +15 Stamina)
Shifting Dreadstone (+10 Agility, +15 Stamina)
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)
Thick King's Amber (+20 Defense)
Stalwart Ametrine (+10 Dodge, +10 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)
Enduring Eye of Zul (+10 Defense, +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 Death Knight By Slot Color
Red
Regal Twilight Opal (+8 Dodge, +12 Stamina)
Defender's Twilight Opal (+8 Parry, +12 Stamina)
Stalwart Monarch Topaz (+8 Dodge, +8 Defense)
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)
Thick Autumn's Glow (+16 Defense)
Stalwart Monarch Topaz (+8 Defense, +8 Dodge)
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)
Enduring Forest Emerald (+8 Defense, +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)
