Orichalcum or aurichalcum is a metal mentioned in several ancient writings, including a story of Atlantis in the Critias dialogue, recorded by Plato. According to Critias (460 – 403 BC), orichalcum was considered second only to gold in value, and was found and mined in many parts of Atlantis in ancient times. By the time of Critias, however, orichalcum was known only by name.

The name derives from the Greek ὀρείχαλκος, oreikhalkos (from ὄρος, oros, mountain and χαλκός, chalkos, copper or bronze), meaning literally "mountain copper" or "copper mountain".

The Romans transliterated "orichalcum" as "aurichalcum," which was thought to literally mean "gold copper". It is known from the writings of Cicero that the metal they called orichalcum, while it resembled gold in colour, had a much lower value.

Orichalcum has variously been held to be a gold/copper alloy, a copper-tin or copper-zinc brass, or a metal no longer known. However, in Vergil's Aeneid it was mentioned that the breastplate of Turnus was "stiff with gold and white orachalc".

In later years, "orichalcum" was used to describe the sulfide mineral chalcopyrite, and to describe brass. However, these usages are difficult to reconcile with the text of Critias,because he states that the metal was "only a name" by his time, while brass and chalcopyrite continued to be very important through the time of Plato until today.

Joseph Needham notes that the 18th century Bishop Richard Watson, a professor of chemistry, wrote that there was an ancient idea that there were "two sorts of brass or orichalcum". Needham also suggests that the Greeks may not have known how orichalcum was made, and that they might even have had an imitation of the original.

In 2015, a number of ingots believed to be orichalcum were discovered in a sunken vessel (in the coasts of Gela in Sicily), which has tentatively been dated as being 2600 years old. Analyzed with X-ray fluorescence by Dario Panetta, of TQ - Tecnologies for Quality, the 39 ingots turned out to be an alloy consisting of 75-80 percent copper, 15-20 percent zinc, and smaller percentages of nickel, and iron.

Numismatics

In numismatics, the term "orichalcum" is used to refer to the golden-colored bronze alloy used for the sestertius and dupondius Roman coins. It is considered more valuable than copper, of which the coin was made.

About the Orichalcum Coin

This is a heavy Sestertius (20.99 grams) of Faustina Senior. 

Obverse is draped bust facing right.  There's a lot of detail on this coin especially Faustina's face.

Reverse is Venus standing facing right holding an apple.     

Empress 138-140 AD
Wife of Antoninus Pius
Died 140 AD

Faustina Senior was the wife of Antoninus Pius. They were married before his accession in 110 AD. Faustina was the daughter of the Consul Marcus Annius Verus and Rupilla Faustina. She was born in Rome prior to the reign of Trajan.

Faustina bore four children, two sons and two daughters. Her two sons, Marcus Galerius and Marcus Aurelius Fulvius, died prior to their father’s accession to the throne. Her eldest daughter, Aurelia Fadilla, was married to Lamia Syllanus, but she had also died prior to 138 AD. Her only surviving daughter, Annia Galeria Faustina (Junior), married the future emperor Marcus Aurelius.

Faustina’s reputation was not altogether pleasant. Her husband went to some lengths in order to stop the rumors. Nonetheless, when she died in 140 AD, Antininous Pius consecrated her in 141 AD. He also issued a very extensive series of commemorative coinage in her memory as well as the dedication of the Temple of Faustina he had erected in the heart of the Forum.

Ancient literature

Orichalcum is first mentioned in the 7th century BC by Hesiod, and in the Homeric hymn dedicated to Aphrodite, dated to the 630s.

According to the Critias by Plato, the three outer walls of the Temple to Poseidon and Cleito on Atlantis were clad respectively with brass, tin, and the third outer wall, which encompassed the whole citadel, "flashed with the red light of orichalcum". The interior walls, pillars and floors of the temple were completely covered in orichalcum, and the roof was variegated with gold, silver, and orichalcum. In the center of the temple stood a pillar of orichalcum, on which the laws of Poseidon and records of the first son princes of Poseidon were inscribed. (Crit. 116–119)

Orichalcum is also mentioned in the Antiquities of the Jews - Book VIII, sect. 88 by Josephus, who stated that the vessels in the Temple of Solomon were made of orichalcum (or a bronze that was like gold in beauty). Pliny the Elder points out that the metal had lost currency due to the mines being exhausted. Pseudo-Aristotle in De mirabilibus auscultationibus describes orichalcum as a shining metal obtained during the smelting of copper with the addition of "calmia" (zinc oxide), a kind of earth formerly found on the shores of the Black Sea.

In popular culture

Orichalcum is often mentioned in a number of high fantasy works and video games of fantasy theme, as one of the more valuable ores, along with fictional mithril. Notable examples include Exalted, Dungeons & Dragons, Shadowrun, Earthdawn, Star Ocean, Golden Sun, The Elder Scrolls, Guild Wars 2, Terraria, and Bravely Default.

Orichalcum is a power source in the adventure game Indiana Jones and the Fate of Atlantis. It is mentioned several times in various entries in the Final Fantasy videogame franchise as well as in Kingdom Hearts II, where it was used as the primary ore in forging the Ultima Weapon. In Kid Icarus: Uprising, Lord Dyntos tells Pit that his vehicle, the Great Sacred Treasure, is made of orichalcum.

Unsolved Mysteries

It has been said from many secret circles that the "other metal" in the alloy is "Sky Iron," also known as meteoric iron from meteorites. This could explain why it was so treasured and why it is only second to gold. Meteorites today are approximately the price of gold too because of their rarity. Most likely back then it was just as rare as it is today and when mixed with other metals made the best alloy known.