Dalekanium

Dalekanium — also spelt Dalekenium, and known to the Thals as Skaronium (AUDIO: Return to Skaro) — was the metal of which Dalek casings were made, (COMIC: The Humanoids, TV: The Dalek Invasion of Earth) as well as ships of the Dalek Fleet. (COMIC: The Only Good Dalek) Dalekanium wire was expensive. (COMIC: Emperor of the Daleks!) Dalekanium could be used as a weapon against the Daleks as well. (GAME: Destiny of the Doctors)

The Forge's archive was reinforced with Dalekanium. (AUDIO: Project: Destiny)

During the Last Great Time War, the Time Lords saw the potential to engineer Vashta Nerada with the ability to consume Dalekanium, planning to unleash them on Dalek battlecruisers and ultimately Skaro itself. However, the project was terminated when the Vashta Nerada broke out on Synthesis Station. (AUDIO: Day of the Vashta Nerada) Similarly, during the Dalek Wars, the human scientist Toshio Shimura developed the Kiseibya as a species of metal-consuming predators with a predilection for Dalekanium, to use as a weapon against the Daleks. The Kiseibya would eat their way through the Dalekanium casings of the Daleks, and lay their eggs inside the Dalek mutants. (AUDIO: Enemy of the Daleks)

Abilities
Dalekanium was bulletproof and immune to most lasers and bombs, making a Dalek hard to defeat. It could attract gamma radiation. Dalekanium panels were attached to the Empire State Building in 1930 to attract gamma radiation, which would introduce Dalek DNA into humans that the Cult of Skaro had captured. (TV: Evolution of the Daleks)

Dalekanium was vulnerable to Nitro-9 and to anti-tank rockets. (TV: Remembrance of the Daleks) as well as Dalek gunstick fire. (TV: The Five Doctors, Revelation of the Daleks, Remembrance of the Daleks, Evolution of the Daleks, Victory of the Daleks, Into the Dalek) It was also seemingly vulnerable to the energy weapons utilised by the Combined Galactic Resistance soldiers aboard the Aristotle, as they managed to destroy at least one Dalek and damaged a few others. (TV: Into the Dalek)

Metalert was a reinforced type of Dalekanium which could better withstand space travel, and was also resistant to most Dalek weapons. First accidentally discovered by Dalek Zeg when his own casing was bombarded with special rays, it was used as the basis for the Daleks' development of their first spacecrafts by the Emperor. (COMIC: Duel of the Daleks) It could also be created by infusing a Dalekanium alloy with Flidor gold and sap from the Arkellis flower, (PROSE: Birth of a Legend) elements also used in the Golden Emperor's own casing. (COMIC: Genesis of Evil) By the beginning of the Last Great Time War, the Arkellis flowers were extinct, and so the rarity of these substances meant that only Dalek Sec, chosen leader of the Cult of Skaro, still had the privilege of a Metalert casing. (PROSE: Birth of a Legend)

Dalekanium was an extremely strong metal, so much so that the Thals could only shape it after first reducing it to an alloy. The Daleks however had the technology to manipulate pure Dalekanium. (AUDIO: Return to Skaro)

Dalekanium was a good conductor. It was able to transfer a Daleks' energy weaponry. (COMIC: Emperor of the Daleks!, GAME: City of the Daleks) The Graak once used a Dalekanium weapon to attack a Dalek. (GAME: Destiny of the Doctors)

Source
Venusian mineralogist Jeff Stone once claimed that Dalekanium was only found on Skaro, indicating it was a natural, rather than synthetic, substance. (COMIC: The Humanoids)

Behind the scenes
Dalekanium — or as it was originally known, Dalekenium — was the earliest concept from spin-off material to make its way into the television programme. It debuted in 1964's The Dalek Book.

In Anti-Dalek Weapons, it was stated that penetron rays had the ability to randomise the molecular structure of Dalekanium, weakening the metal on a macroscopic level — an effect exploited by a Venusian anti-Dalek weapon, the metal fatigue inducer.