Gwaith-i-Mírdain

The Gwaith-i-Mírdain was a brotherhood formed by Celebrimbor and the other master craftsmen of Eregion in the Second Age and remained close to the Dwarves of Khazad-dûm and their mithril mines. They cooperated for about a thousand years, and their rare friendship was the closest between Elves and Dwarves in history. Celebrimbor helped to construct the famous and magical West-gate of Moria. They were said to be the most talented artisans to have worked since the time of Fëanor himself (who was Celebrimbor's grandfather).

Their skill was subverted by Sauron, in the guise of Annatar the "Lord of Gifts" in S.A. 1200, who focused their abilities on forging the Rings of Power in S.A. 1500 and indeed Celebrimbor present King Durin III with a Ring of Power. The Rings were completed in S.A. 1590, when Sauron betrayed them and forged the One Ring. However the Gwaith realized his plan and denied to hand him their rings, resulting in the War of the Elves and Sauron. Eregion was devastated and the Gwaith retreated to Rivendell

Etymology

Gwaith-i-Mírdain means "Brotherhood of Jewel-smiths" in Sindarin (from gwaith = "host, people" and mírdan = "jewel-smith").