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

While other gems give higher resistances to individual source of damage, diamonds give easily the most total resistance. Placing diamonds in socketed shields are a very easy, yet very powerful way of adding resistances. In Armor and Helms, it adds an increase in Attack Rating.

In Shields, it adds bonuses to all Resistances, reflecting the fact that white is composed of all colors and by extension all elements. In Weapons, it adds a bonus to damage against Undead enemies, an indication of its relation to the Light. The white in a Diamond signifies Purity and the Power of the Light. Like all the other Gems, Diamonds grant bonuses that are related to their color. They are white/transparent, and can be socketed into items.

Diablo 3 gem levels