Dalekanium

Dalekanium — also spelt Dalekenium — was the metal of which Dalek casings were made. (COMIC: The Humanoids, TV: The Dalek Invasion of Earth) 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)

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. The casing was infused with Flidor gold and sap from the extinct Arkellis flower. The rarity of these substances meant that only Dalek Sec used this substance. (PROSE: Birth of a Legend)

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.