The Silithar (pl. -air) is an adamant made up especially for Lotro and is not mentioned in Tolkien's works. It does, however, get its inspiration from the powerful stones crafted by elves of the First Age and dwarves who learned the skills from elves.
There are no shortage of tales about the renowned sword named Narsil. However, regarding the Silithair we may only find information from within Lotro: from quests, some quotes, and from the developers:
These adamants are assumed to have been crafted by elves of the house of Fëanor, and possessing the light of the stars. One of these Silithair were used by Telchar, the legendary Dwarven-smith, to forge the Narsil. This is the sword that cut off the One Ring from the hand of Sauron and it begun its peculiar journey. It all begun by the time of Elendil's death and by the hands of Isildur, his son. Eventually the shards of the sword, and its Silithar, was brought back to Valandil, son of Isildur, in Tinnudir. The Silithar was put down in Elendil's symbolic tomb, Haudh Elendil, at Annúminas, and is long since drowned by the powers of Gwindeth to protect it from unrighteous hands.
See the quest chain The Blade That Was Broken for the full story line.
From quests and NPCs:
- Of old, Elves of the house or Fëanor crafted numerous wonders, items of great power and beauty. Among these were the Silithair, adamants possessed of the light of the stars. The dwarf-smith Telchar used one of these stones in the creation of Narsil, and the Silithar it was that caused the blade to glow with silver light when wielded in battle.
- Elendil never returned from the wars to which he marched in the South, but the Silithar came by various keepers to Valandil his heir, who placed it in the deepest chamber of the tomb.
- The adamant you seek, the Silithar that passed from Gil-galad to Elendil and his line through the safe-keeping of Círdan the Shipwright, cannot be reclaimed without my permission. It was the will of Amlaith that the Silithar be kept safe, even from those of Elendil's line, for fear that its light would be wasted unduly or the stone taken by the Enemy.
- ..the Silithar that lay within Annúminas -- the only one of the stones to remain in this bend of the world..
- All of the Silithair were lost during the long centuries since the forging of Narsil. Without such a stone to use in the reforging, the sword will never achieve the greatness it once had.