Amethyst

Amethysts are purple gems, as well as socketable items. Like all the other Gems, Amethysts grant bonuses that are related to their color. The purple in Amethyst symbolizes Endurance and Vitality.

Diablo II
In Weapons and Shields, Amethysts add bonuses to Attack Rating and Defense, a sign of the perfect warrior and in Armor and Helms, it adds a direct increase in Strength, a factor that is used to indicate the Endurance of a person. Crafted Items can be made with Amethysts, and Caster Items have bonuses centered on mana and magic use.

Weapons: Adds to Attack Rating

Shields: Adds to Defense

Helms and Body Armor: Adds to Strength

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

Trivia

 * In certain traditions, Amethysts are valued as stones of concentration and defiance: Buddhist monks used them in meditation, and ancient Greeks believed it protected against evil and violent death. In game, certain elements of these beliefs are reflected in Amethyst's boosts to defense (protection), attack rating (concentration), and strength (confidence).