The Diablo 2 Wiki
The Diablo 2 Wiki archive is available for anyone looking for information on Diablo 2. All information here is pre-D2R but contains a lot of useful information that is still relevant. Updated sections for new D2R features can be found on the PureDiablo Diablo 2 section

Diamond

From Diablo 2 Wiki

Diamonds are among the most useful of gems in the game, very popularly socketed in shields for the resistance bonus.


Diablo III Diamonds

Diamonds are not found in Diablo III. Only Rubies, Topazes, Emeralds, and Amethysts are found in D3C, though other gem types might return in expansions. See the Diablo III wiki for more details.


Diablo II: Lord of Destruction

The following bonuses are provided by socketed Diamonds in D2X.

Quality Weapon Helm or Armor Shield Clvl Req
Chipped Diamond-chipped.gif +28% damage vs. the undead +20 to Attack Rating +6% Resistance to All 1
Flawed Diamond-flawed.gif +34% damage vs. the undead +40 to Attack Rating +8% Resistance to All 5
Normal Diamond-normal.gif +44% damage vs. the undead +60 to Attack Rating +11% Resistance to All 12
Flawless Diamond-flawless.gif +54% damage vs. the undead +80 to Attack Rating +14% Resistance to All 15
Perfect Diamond-perfect.gif +68% damage vs. the undead +100 to Attack Rating +19% Resistance to All 18


Diablo II Classic Stats

Diamond stats in D2C were slightly lower than they are in the expansion.


Diamond

Quality Weapon Helm or Armor Shield Sell Price Clvl Req
Chipped Diamond-chipped.gif +28% damage vs. the undead +10 to Attack Rating +5% Resistance to All 125 1
Flawed Diamond-flawed.gif +34% damage vs. the undead +15 to Attack Rating +7% Resistance to All 375 5
Normal Diamond-normal.gif +44% damage vs. the undead +20 to Attack Rating +10% Resistance to All 1250 12
Flawless Diamond-flawless.gif +54% damage vs. the undead +25 to Attack Rating +14% Resistance to All 3750 15
Perfect Diamond-perfect.gif +69% damage vs. the undead +30 to Attack Rating +19% Resistance to All 7500 18