From the writings of the deceased sage Elminster…
Let’s get this out of the way: ‘Adamantine’ is an adjective. ‘Adamantite’ is an ore. ‘Adamantium’ is the finished product.
Fact: The hardest metal is adamantium.
Adamantium isn’t just hard the way that steel is “hard”, it is hard the way that the Pythagorean theorem is true. Once a piece of adamantium has cooled, its atoms are (nearly) immovable with respect to each other.
Most items made of adamantium are thinner than you might think, since a small piece is just as strong as a large piece. The size of adamantine items has more to do with the difficulty of forging them.
Adamantite ore is found only in starfalls. Once it has been forged, the metal is beyond strength. Many speculate that the metal becomes lost to time, impervious to any change at all. An adamantium object will survive the heat death of the universe. When the solar system collapses into a black hole, it will be covered with adamantine swords.
Speaking of adamantine swords: the best adamantine swords are mostly made from steel. Only the merest sliver near the cutting edge is made from adamantium, since an adamantium toothpick is as strong as adamantium girder. The steel is only there to give it weight, since a featherweight sword is still a poor cutting instrument.
The cutting ability of an adamantium weapon is decent, at best. Since the blade can never be sharpened, the edge can only be honed prior to quenching. No one is certain how the ancient blacksmiths sharpened their adamantium blades to such a fine point, but that information would be worth a fortune if it were rediscovered in some ancient dungeon.
Adamantium makes for better armor than weapons, although it is defeated by its cost. Any king who can afford a suit of adamantine armor can also afford an army—and that king is much better served by the army.
Because of this, newly claimed adamantite is usually destined for architecture or furniture.
There are a few different ways to forge adamantium. The most commonly known way is use one of the dwarven lightning forges, a task that requires as much diplomacy as luck. There is a well-known forge among Clan Griff, but securing the location is difficult. A finished item of adamantium can never be melted down, reforged, or remade in any meaningful way short of a Wish or similar magic.
It is also a perfect insulator, but few people care about the insulating properties of adamantium, since it isn’t very relevant to swords. Still, the thermal sensation of adamantium is closer to styrofoam than steel.
It is commonly believed that adamantite meteors are actually fragments of the original sun, which displeased Ao with its blasphemy.