From Diablo Wiki
Emeralds are one of the five types of gems found in Diablo III. The others are Amethyst, Ruby, Topaz, and Diamonds, the last added in Patch 2.0.1 and Reaper of Souls. See the main gems article for full details about gem scarcity, upgrade recipes, strategy, and more.
Emeralds boost Dexterity when socketed in armor, add Critical hit Damage in weapons, and Gold Find in helm.
Emeralds is generally the most popular gem to socket into a weapon in the end game, since well-geared characters, especially those with high Critical hit Chance, get the most benefit out of raising their Critical hit Damage. Emeralds are much less useful in weapons for lower level or poorly-geared characters who do not have at least 25% ChC.
Emeralds are also used by Monks and Demon Hunters to boost their dexterity and damage in armor. Helms are the only non-viable socketing option for Emeralds, since almost every player feels that there are many better choices to fill the hole in their head. It has often been suggested that the Emerald in helm bonus be doubled or tripled (or more) to make that a more viable option.
During the Beta test, Emeralds boosted Magic Find in helms. This bonus was switched back to Topaz (as it was in Diablo 2) before Diablo 3's release and Emeralds became the helm Gold Find gem.
|Chipped Emerald||+5% Increased Gold Find||+6 Dexterity||Critical Hit Damage Increased by 10%|
|Flawed Emerald||+7% Increased Gold Find||+10 Dexterity||Critical Hit Damage Increased by 15%|
|Regular Emerald||+9% Increased Gold Find||+14 Dexterity||Critical Hit Damage Increased by 20%|
|Flawless Emerald||+11% Increased Gold Find||+18 Dexterity||Critical Hit Damage Increased by 25%|
|Perfect Emerald||+13% Increased Gold Find||+22 Dexterity||Critical Hit Damage Increased by 30%|
|Radiant Emerald||+15% Increased Gold Find||+26 Dexterity||Critical Hit Damage Increased by 35%|
|Square Emerald Emerald||+17% Increased Gold Find||+30 Dexterity||Critical Hit Damage Increased by 40%|
|Flawless Square Emerald||+19% Increased Gold Find||+34 Dexterity||Critical Hit Damage Increased by 45%|
|Perfect Square Emerald||+21% Increased Gold Find||+38 Dexterity||Critical Hit Damage Increased by 50%|
|Radiant Square Emerald||+23% Increased Gold Find||+42 Dexterity||Critical Hit Damage Increased by 60%|
|Star Emerald||+25% Increased Gold Find||+46 Dexterity||Critical Hit Damage Increased by 70%|
|Flawless Star Emerald||+27% Increased Gold Find||+50 Dexterity||Critical Hit Damage Increased by 80%|
|Perfect Star Emerald||+29% Increased Gold Find||+54 Dexterity||Critical Hit Damage Increased by 90%|
|Radiant Star Emerald||+31% Increased Gold Find||+58 Dexterity||+100% Critical Hit Damage|
|Marquise Emerald||+33% Increased Gold Find||+62 Dexterity||+110% Critical Hit Damage|
|Imperial Emerald (RoS only)||+35% Increased Gold Find||+100 Dexterity||+115% Critical Hit Damage|
|Flawless Imperial Emerald (RoS only)||+37% Increased Gold Find||+160 Dexterity||+120% Critical Hit Damage|
|Royal Emerald (RoS only)||+39% Increased Gold Find||+220 Dexterity||+125% Critical Hit Damage|
|Flawless Royal Emerald (RoS only)||+41% Increased Gold Find||+280 Dexterity||+130% Critical Hit Damage|
Diablo II Emerald
Though, their stats are not the same as Emeralds in Diablo III. The following are the stat bonuses for Emeralds in Diablo II: Lord of Destruction.
|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|
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