Emerald

Emeralds are green socketable gems.

Diablo II
Like all the other gems, Emeralds grant bonuses that are related to their color. The green in Emeralds signify the Elemental power of Poison and Pestilence. In Weapons and Shields, it adds bonuses related to Poison and in Armor and Helms, it adds an increase to Dexterity. Crafted Items can be made with Emeralds, and Safety Items have bonuses centered on defensive stats.

Weapons: Adds Poison Damage

Shields: Adds to Resistance to Poison

Helms and Body Armor: Adds to Dexterity

Development
Prior to patch 2.3, Emerald gems in Diablo III had many more tiers, but were condensed for easier crafting.

Trivia

 * In Roman mythology, the Emerald is traditionally the symbol of Mercury, God of speed, wit, and travel. This belief is perhaps reflected in game, where Emeralds give bonuses to Dexterity.