Diamond

Diamonds are gems. They are white/transparent, and can be socketed into items.

Diablo II
Like all the other Gems, Diamonds grant bonuses that are related to their color. The white in a Diamond signifies Purity and the Power of the Light. In Weapons, it adds a bonus to damage against Undead enemies, an indication of its relation to the Light. In Shields, it adds bonuses to all Resistances, reflecting the fact that white is composed of all colors and by extension all elements. In Armor and Helms, it adds an increase in Attack Rating.

Placing diamonds in socketed shields are a very easy, yet very powerful way of adding resistances. While other gems give higher resistances to individual source of damage, diamonds give easily the most total resistance. A perfect diamond gives +19% to all resistances, something that is only beaten by an Um Rune, which is considerably more difficult to obtain and only gives 3% more resistance in a shield.

The Diamond has a different sprite for the 'normal' tier. It is diamond shaped instead of the normal rectangular shape for the other 5 gems.

Weapons: Adds to Damage vs. Undead

Shields: Adds to all Resistances

Helms and Body Armor: Adds to Attack Rating

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

Trivia

 * Many traditions in the world reference the Diamond's affinity for light, and Roman philosopher Pliny the Elder referred to its "divine brilliance on earth." The brilliance of a diamond due to its high refractive index is symbolized in game, as their Prismatic nature banishes evil (in this case, undead, which are described in many stories as being harmed by light, especially sunlight and light-based magic) and protects against all elements.