Adamant

Adamantine is a term used to describe any material which composition was that of a very hard material.

Myths & Legends
Although not only referring to metals, Adamantine was also known as the metal of the gods.

Adamantine is described as being unbreakable, untarnishable, lightweight and resilient.
 * Perseus used an adamantine sword to decapitate the Gorgon Medusa.
 * Presumably, Adamantine was the material used by the gods to chain Prometheus.
 * the gates on Tartaros are said to be guarded by a massive gate made of Adamantine.

Geology
In Geology and in Manufacturing enginnering, Adamantine is a mineral, often referred to as adamantine spar. It is a silky brown form of corundum. Adamantine is also used as an adjective to refer to non-metallic, brilliant light reflecting and transmitting properties, known as adamantine luster.

Comics
Adamantine_wolv.jpg|Adamantine in Astonishing X-Men 
 * On the Astonishing X-Men's storyline Exalted, Cyclops meets alternate versions of his X-Men teammates, one of them is an alternate Wolverine, whose inner skeleton and therefore claws are composed of Adamantine instead of Adamantium.