A rare ore, that is difficult to come by, and only a skilled worker can make into armor and weapons.
It’s a jet-black metal that shimmers green under torchlight and purplish-white under magical light.
Melee weapons and ammunition made of or coated with adamantine are unusually effective when used to break objects.
Whenever an adamantine weapon or piece of ammunition hits an object, the hit is considered to be a critical.
Finding a smith who can successfully work with adamantine is likely to be no small feat in and of itself.
While adamantine weapons are not inherently magical (a +1 adamantine longsword is just a very good sword), they are still effective in overcoming damage resistances displayed by some creatures – specifically constructs.
Animated statues, gargoyles, stone golems, and their ilk commonly have resistance (or even immunity) to all Bludgeoning, Piercing, and Slashing damage from nonmagical sources.
But many of these monsters, like the Giant Four-Armed Gargoyle, specifically note that adamantine weapons overcome these resistances.