Atium is one of the two god metals. An Allomancer burning atium is able to see into the future by a few seconds. Feruchemists can use atium to store age, and when used as a Hemalurgic spike, atium steals any property or power. The metal is a reflective, silvery colour, so much so that beads of atium almost appear to be drops of liquid.
Presumably, the name "atium" is inherited from Ati, the name of the original holder of the Shard Ruin, just as the name "lerasium" is inherited from Leras, the original holder of the Shard Preservation.
An atium Misting is known as a Seer. Burning atium allows the user to see a few seconds into the future, by producing atium shadows of everything they can see. The shadows perform future actions. Burning atium expands the user's mind to be able to accept and process all of the information. If another Allomancer burning atium is encountered, the atium shadow will split, since each seeing into the other's future means that they can both make different decisions that affect both Allomancers' outcomes. An Allomancer's atium shadow will also split if they are burning electrum.
Alloys of atium produce various expanded mental and temporal effects.
Atium, when used by a Feruchemist, stores age. When storing age, the Feruchemist spends time older. When tapping age, the Feruchemist then spends time younger.
If the Feruchemist is also either an atium Misting or a Mistborn it is then possible to burn an atiummind to get an enormous burst of youth - allowing for practical immortality.
When used as a Hemalurgic spike, atium is like a wild card. Depending on where it is placed, it can steal any Hemalurgic property and it does so better than the original spike.
Atium may also be alloyed with any of the other Allomatic metals. However the only currently known atium alloy is malatium an atium-gold alloy. These alloys grant various temporal effects when burned.
Atium is different from other Allomantic metals, in that when it (and presumably its alloys) is burned, it provides the power rather than Preservation, since it is of Ruin. 
- The Final Empire chapter 13
- Mistborn Adventure Game chapter 15
- The aluminum would not give an atium shadow.
— Theoryland - March 6th, 2014
- Why does Atium steal temporal powers?
— Theoryland - 2009-07
- What would an atium-electrum alloy do in Allomancy?
— Theoryland - 2010-12-06
- Annotation for The Hero of Ages chapter 71
|The Metallic Arts|
|Arts||Allomancy · Feruchemy · Hemalurgy|
|Concepts||Metalborn · Snapping · Savants · Metalminds · Twinborn · Compounding · Blessings|
|Constructs||Steel Inquisitors · Koloss · Kandra|
|Base Metals||Iron · Steel · Tin · Pewter · Zinc · Brass · Copper · Bronze · Cadmium · Bendalloy · Gold · Electrum · Chromium · Nicrosil · Aluminum · Duralumin|
|God Metals||Atium (Malatium) · Lerasium · Ettmetal|
|Era 1 books||Mistborn: The Final Empire · The Well of Ascension · The Hero of Ages · (Mistborn: Secret History · The Eleventh Metal)|
|Era 2 books||The Alloy of Law · Shadows of Self · The Bands of Mourning · The Lost Metal · (Allomancer Jak and the Pits of Eltania)|
|Shards||Preservation · Ruin · Harmony|
|History||Classical Scadrial · Final Empire · Catacendre · Elendel · Southern Scadrian|
|Magic||Metallic Arts (Allomancy, Feruchemy, Hemalurgy) · Metalborn · Mistborn · Misting · Savant · Ferring · Twinborn · Compounding · Atium · Lerasium · Ettmetal|
|Other Media||Mistborn Adventure Game · Mistborn: Birthright · Mistborn film · Mistborn: House War|