1 Lucas: Here’s the adamantite door to the throne room. The only way to open it is to insert swords into the three slots.
2 Eleven: But we don’t have any swords.
3 Mike: We have flashlights. We can use some metal from them. Anything conductive should work.
4 Lucas: Wait. We could have used metal strips when we played this adventure?
4 Mike: Sure. If you had flashlights!

In the official adventure description of Tomb of Horrrors, the door is described as "adamantite", with no explanation of what that is. If you read the Advanced Dungeons & Dragons rule books carefully, you can find a sprinkling of other mentions of this mysterious substance.

The Players Handbook has exactly one mention of adamantite, in Appendix I on Psionics, an entirely optional set of rules that most players never used. Under the description for the "Major Science" (i.e. psionic power) of Molecular Rearrangement (p. 115), it lists types of metals that can be transmuted, specifying that the difficulty depends on the hardness of the metal. The easiest level lists gold and lead "and others very soft". Next comes copper, silver, tin, zinc, et. al. Then platinum, nickel, iron, et. al. Then steel. Then Mithril (the legendary exotic dwarven metal mentioned in The Lord of the Rings) and steel alloys. And finally, alone at the most difficult level: adamantite.

The Dungeon Masters Guide contains four mentions of adamantite. Three are simply to say that some specific magic items (Daern's instant fortress, p. 142, saw of mighty cutting. p. 153, and talisman of the sphere, p. 155) are made of this material, with no further embellishment. The fourth is to state that magical swords of +5 quality (the best) are made of adamantite alloyed steel (p. 164).

The Monster Manual contains two references. The demon lord Yeenoghu (p. 19) carries a magical flail "with three chains of adamantite, each bearing a spiked ball". And the rust monster (p. 83) is stated to be able to corrode and eat steel alloys "such as mithril and adamantite".

This is the entirety of the mentions of this material in all of the core rule books. If you were a diligent reader, you could get some idea of what this material was meant to be like and its properties, but nothing in any detail, or any rules regarding how easy or difficult it is to work with or damage, or how mundane or magical it is, other than that it's apparently harder and stronger than any other metal.

Which, to be fair, is probably enough for game purposes.