Emerald

Emeralds stack oddly, adding much more damage than you would expect. When multiple emeralds are socketed, the duration adds, and the damage is squared: 2 = 4x damage over 2 seconds, 3 = 9x damage over 3 seconds, etc. Adding other types of poison damage from jewels in armor/helms/shields, poison charms, unique armor, etc can create considerable, long-lasting damage.

Quality	Weapon	Helm or Armor	Shield	Clvl Req
Chipped	Adds 5 (5.9) poison damage in 1 sec	+3 to Dexterity	+12% Poison Resistance	1
Flawed	Adds 11 (11.7) poison damage in 1 sec	+4 to Dexterity	+16% Poison Resistance	5
Normal	Adds 17 (17.6) poison damage in 1 sec	+6 to Dexterity	+22% Poison Resistance	12
Flawless	Adds 23 (23.4) poison damage in 1 sec	+8 to Dexterity	+28% Poison Resistance	15
Perfect	Adds 29 (29.3) poison damage in 1 sec	+10 to Dexterity	+40% Poison Resistance	18