In the first edition Dungeon Master's Guide on page 164, it is mentioned that all +3 weapons are meterorite iron steel, all +4 weapons are mithral alloyed steel and all +5 weapons are adamantite alloyed steel. I like the idea of specific metals being requirements for enchanted weapons and armor (and other items as well) but why limit it to just those three? Here are a dozen materials and metals that can be used for specific pluses as well as some with side effects. One references a 2nd edition idea, otherwise all can be used in either edition of AD&D.

Dragon Clot. When an ancient or older dragon dies and is left to rot on soil, its blood leaks into the ground and congeals. Over time, the blood hardens and absorbs the ambient magic of the environment. After a century, it becomes hard as steel and can be used to make +3 weapons and armor. Dragon clot weapons do double damage to the dragon species they came from and armor increases the wearer's saving throw versus that species of dragon's breath weapon by +4. Gnomes are the people who most commonly mine and use dragon clots as dwarves prefer to avoid earth and mine deep into stone.

Typhoon Whale Bone. Typhoon whales are living disasters that are always in the eye of a cyclonic storm. If one braves the sea and slays such a beast, they can mine the corpse for many things. Their blubber can be used for boosting weather control spells, their eyes can see into the planes of water and air and their bones can be used to make +4 weapons. Such weapons do double damage to fire and earth elementals.

Sky Lance. Some storms go far beyond the sky on alien worlds and planes. Lightning strikes from moons, suns or stars can result in devastation and sky lances- pure alien metal in the shape of a spike or lance buried where the strike occured. They can be used to make +2 weapons and armor, but their real value is in making rods. Those rods made from sky lances have 50% more charges.

Salt From the First Sea. Oddly this is always found deep underground rather than on the bottom of the ocean. The material can be used to make +3 weapons that dehydrate opponents, doing an additional 1d4 (or 1d8 for water elementals and plants) points of damage.

Elder Brain Mucus. Illithid elder brains are covered in slime. If treated by an alchemist, it can be turned into a metal like substance as hard as steel. The result can be turned into +5 weapons that do double damage versus psionic creatures and triple versus illithids.

Metal Plague Victim. This disease turns people and animals into metal statues. Each organ system becomes a different metal. In the correct combination, such metal can be turned into +2 weapons and armor. If made from a person, the wielder gains one language from the victim. If made from an animal and turned into armor, the wearer gains a number of hit points equal to the anima's hit dice.

Pages from the Book of Life. This infinitely long book is hidden by the gods. If discovered, some of the blank pages can be removed and used to make +3 armor that also provides 80% magic resistance (50% in 2e). Taking pages with writing is a good way to die quickly when the gods find out.

Fossilized Elder Treant. The age of this fossil is up to the DM, but I would have it at least 10 million years. The stone can be used to make +4 weapons that do triple damage versus undead and aberrations (in the 3e sense).

Cloud Silver. Found only in the clouds inhabited by cloud giants and silver dragons, cloud silver can be used to make +2 weapons and armor. The weapons do double damage to lycanthropes and devils and the armor does 1d4 points to any lycanthrope or devil (or undead vulnerable to silver) that touches it.

Black Oak. When an oak over 500 years old is struck by lightning, there is a 1% chance that one of the branches becomes black oak, a hardened material. It can be used to make +4 weapons that does double damage versus aberrations (in the 3e sense). Non-druids that grasp black oak feel ill and those who touch enchanted black oak take 1 point of damage per turn (not round).

Alchemical Tin. After being purified by an alchemist, tin can gain a +3 enchantment. It can not corrode and weapons made from it are unaffected by water. In other words, they can be used as easily underwater as above it.

Alchemical Iron. This metal can gain a +5 enchantment and like alchemical tin, never corrodes. It does double damage to demons.

Another source you might find useful is Dragon 194. It has an article on "Bonded Weapons", weapons made from fear, the elements, pain and similar things by the gods. They could easily be used in either edition of AD&D, though they may have some conversion issues for 3e as they are artifacts.