Orichalcum (also called Orichalc) is a grayish-green metal used primarily to make weapons and armor. Orcs are famed for their use of this metal, with some comparing its qualities to the Orcs themselves - green, exceptionally rigid, stronger than steel, and extremely temperamental.
Also during the Merethic Era was the legend of Diagna, an avatar of the HoonDing. Diagna was said to have brought orichalc weapons to the Yokudans during their war with the Lefthanded Elves, granting them victory. Thereafter Diagna became known as the Orichalc God of the Sideways Blade. 
Orcish smiths during the the First Era became very familiar with orichalc, and were believed to wielded tools even larger than those used by Orcs in modern times. This continued in the following eras, where orichalcum arms and armor became synonymous with the Orcs.
While orichalcum could be found almost anywhere in the mid-Second Era, miners near Wayrest made the biggest orichalcum strike in recorded history in 2E 561. With King Gardner of Wayrest's approval, Earl Emeric of Cumberland proposed to use the resulting wealth from Cumberland Mine to enhance Wayrest's fleet and improved trade throughout High Rock. Such was prosperity that the king gifted orichalcum pails to the region's prominent farmers.
Orichalcum is often found in brittle shale, making excavation somewhat easier than other metals. The deposits also tend to be free of gangue, making ore dressing unnecessary. While extraction can be relatively striaghtforward, orichalcum is notoriously difficult to smelt. A consistent low heat must be maintained to prevent the metal from becoming brittle and cracking. Orcs are said to be the masters of this technique, although any sufficiently skilled smith can learn their methods.
Some smiths have been known to blend iron into the mix to act as a binding, but many Orcish smiths dismissed this method as evidence of laziness or lack of skill. According to the famed smith Gurzbog gra-Gulog, pure orichalcum is far lighter and stronger than its iron-alloyed alternative. This difference in weight is important given the Orcs' reliance on massive plate armor.
Orichalcum weapons retain their sharpness far longer than most weapons made with conventional metals, and they are remarkably resistant to rust and chipping. Some Orcish smiths remark that its unique color is also noteworthy, making arms and armor less gaudy as High Elven glass and not as drab as iron or steel.
Arms and armor are not the only uses for the green metal - uses range from household items, tools, statuary and even alchemical research. In ancient times, Yokudans statuary was known to feature a metallic element made of orichalcum. Hearkening back to Diagna's intervention in the war with the Lefthanded Elves, Yokudans were also known to create Orichalc statuettes depicting Diagna holding a severed head.
- Notes on Orichalcum — A short essay on the properties of Orichalcum
- Orcish equipment has appeared in TES II: Daggerfall, TES III: Morrowind, TES Travels: Shadowkey, TES IV: Oblivion, TES V: Skyrim and The Elder Scrolls Online. Orichalcum has only appeared in Skyrim, ESO, and Blades.
- Notes on Orichalcum
- The Shattered Staff quest in ESO
- Varieties of Faith... — Brother Mikhael Karkuxor of the Imperial College
- Anvil of Old Orsinium antiquity codex entry in ESO
- Orcish weapons and armor in ESO, Morrowind, Oblivion and Skyrim.
- Guide to the Daggerfall Covenant
- Triumphs of a Monarch, Ch. 10 — His Majesty King Emeric
- Orichalcum Milking Pail contraband item in ESO
- Heavy Armor Forging — Sven Two-Hammers
- Orichalipers contraband item in ESO
- Orcish Hairbrush contraband item in ESO
- Orichalc Kurog Bust contraband item in ESO
- On the Tel Var Stones: Volume 3 — Herminius Sophus
- Morwha's Blessing antiquity codex entry in ESO
- Orichalc Diagna Statuette contraband item in ESO
Note: the following references are not from official sources. They are included to provide a rounder background to this article, but may not reflect established lore.