I use various metals in my campaigns. I have written at length about silver for weapons, here, here, and here. The gist of all of these posts is silver is a special metal, a magical metal. In my campaign, it is a gateway metal for true magical weapons. By its very properties, it can strike targets immune to regular weapons but does not confer bonuses to hit or damage. It is the metal of choice for fey folk and elves, be it for weapons or armor.
Iron is also a special material. Iron poisons fey folk, so they will not use it. They also look down on those that do use it. Regular weapons are usually made of some sort of steel, so the more sophisticated cultures don't carry iron weapons anymore. The fey receives 1 extra point of damage from a steel weapon, but actual iron weapons do an additional 1d4 points of damage, dissipating at a rate of 1 point per round.
For example, if struck by a short sword made of iron, a pixie would take 1d6 from the weapon itself and 1d4 for the iron's poisoning effect. If 4 points of damage were rolled for the iron poisoning, it will dissipate over 4 rounds. 4 HP in the first round, 3 in the next, and so on.
If crafted into armor, it will do poison damage to fey folk just like a weapon - 1d4 HP dissipating. This is per touch. Steel armor will not cause any damage as it can't penetrate the skin and get into the blood.
Elves do not use iron for two reasons: they are too advanced for it and they respect fey folk too much. They also have mithril, which is finer than steel or silver.
The next special metal in my campaign is Cold Iron, also known as meteoric iron. It is exceeding rare. It is always at least +2. If a creature is from a different plane, the bonus is doubled and it also causes a poisoning effect just like iron does to the fey.
Things look grim for magical folks in my campaigns. No so. They have access to Blood Metal. Blood Metal is a dull blue color, and takes a fine edge. Blood Metal affects mammals, snakes, birds, and lizards (but not spiders, insects, or magical beings) with draining damage. Every strike causes a save vs. paralyzation. If failed the target is weakened to the point of exhaustion. This is dastardly metal when added to sling bullets or arrowheads.
Demon's Bane is wrought iron alloyed with many different kinds of minerals and metals in a magical process. This kind of weapon has a bonus to hit but never a bonus to damage. The maximum bonus to hit is +5. Sometimes, precious metals are applied to the surface. All weapons and armor made of Demon's Bane appear to be ceremonial. Against creatures of the prime material plane, they do half the damage and never apply their bonuses to hit.
However, when used against creatures from other planes they receive the bonus to hit and it does normal damage. People wearing Demon's Bane armor cannot be touched by creatures of other planes or those who are astral or ethereal. However, wearers can be struck with a weapon.
When used on a Demon (not a devil) this metal shows its worth. Demon's Bane causes a Demon's blood to boil and burn. This causes an additional 1d8 points of damage dissipating over up to 8 rounds. This is in addition to the damage done by the weapon itself. Armor and holy symbols also cause the same 1d8 point burning damage on contact despite not normally being able to cut or injure a Demon like a weapon can.
Now one final point in this post. I have a schedule for when a character can draw and hold a magic weapon. For AD&D it was:
Silver 1-3 levels
+1 4-6 levels
+2 7-9 levels
+3 10-12 levels
+4 13-15 levels
+5 16+ levels
For OSE and rules which do not get to such heights, every other level suffices. The reason for this rule is I could give a character a plussed weapon on day one and just not think of it again. It would work like a special survival reward for reaching x level. Usually, the pitch was this was a family weapon passed from parent to child and was only effective in the hands of the worthy.
