Armadura
O Guia do Mestre Masmorras fala sobre o Armor na página 150.
Adamantine Armor
Armor (medium or heavy, but not hide), uncommon
This suit of armor is reinforced with adamantine, one of the hardest substances in existence. While you're wearing it, any critical hit against you becomes a normal hit.
Armas
Há mais informações sobre Armas Adamantinas no Guia de Xanathar para Tudo na página 78:
Adamantine is an ultrahard metal found in meteorites and extraordinary mineral veins. In addition to being used to craft adamantine armor, the metal is also used for weapons. 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 a critical hit.
The adamantine version of a melee weapon or of ten pieces of ammunition costs 500 gp more than the normal version, whether the weapon or ammunition is made of the metal or coated with it.
Assim, afeta a dureza, peso, custo e dano. A armadura adamantina transforma golpes críticos contra o usuário em golpes regulares; qualquer golpe de uma arma adamantina contra um objeto causa dano extra porque é um acerto crítico.