Silver and Steel

It's a common fantasy trope that weapons (especially blades) made of certain materials have different effects versus various uncommon and supernatural beings. (Silver being the bane of werewolves is classic, or "cold iron" vs. various fey beings. The Witcher series of videogames, for example, capitalizes on these conventions by arming the protagonist with both an iron sword and a silvered sword, each useful for various foes.)

Anyway, here's how I flesh out that trope in the games that I run.
(This is the kind of post that I may add to and maintain over time.)


Silver
"When the Cosmomachy rent the archaic celestial order, a third of the Archons were hurled from the Heavens for their rebellion, falling to Earth and hiding themselves beneath Her skin to escape the burning Sun. It is said that their mighty fall - tremendous impacts rending and scarring the Earth's surface - caused their revoked glory to slough and shed and become buried in the rock they shattered. Now bereft of grandeur and holiness, these Fallen Archons still infest the subterranean depths of this world, no longer Archons, but Chthons. Their lost glory still remains, however: it forms the veins of silver that we mine for jewelry and coin. Furthermore, this silver still bears all the force and authority of the Chthons' exile: it repels, rends, and burns with wondrous potency any Chthon, their dark spawn, or any being tainted by the Powers Beneath the Earth. Those who venture into the deep places of the world do well to carry a weapon treated with a silver coating, for such precaution may well make the difference between life or death for one who meets a creature tainted or created by a Chthon - or, Heavens forbid, a Chthon itself. As silver is not a strong metal, however, the coating of such weapons may be damaged or rendered useless by even one intense encounter."

Iron
"Esteemed mystics teach that if silver is the lost glory of the Chthons, iron is their excrement, left in the Earth's bones for us to dig up and fashion into cruel weapons for the shedding of blood. While many scholars (such as myself) hesitate to rush into such an assertion, it is true that wrought iron seems to affect Heavenly beings in a strange manner - its touch is reported to chill and repel them as if it were a supreme blasphemy. Fey beings of the upper Earth seem similarly affected."

Steel
"Although, as we have postulated, iron has unexpected effects upon creatures of Heaven and the upper Earth, steel lacks this special character - perhaps the repeated heating and pounding of the forge drive the impurity from it. In any case, good-quality steel seems to have no special character in the way that iron and silver do, but is, of course, simply stronger and more durable than wrought iron, and so much more preferable for most purposes, though its manufacture is still difficult and time-consuming. Indeed, the crafting of steel implements seems to have been the sole domain of the elven peoples as lately as the Northcoming of Numaris, and may even have been lost entirely as a result of that event, only to be rediscovered within the past several centuries. Several cities of the Inner Six claim responsibility for this rediscovery, and it is unclear which claim, if any, is true."

Elfsteel
"The greatest of elfsmiths learned how to forge meteoric iron alloyed with silver into blades as strong as steel, but with the chthono-antagonistic properties of purest silver. (It is thought that meteoric iron, as opposed to chthonic iron, lacks the unholy origins of such base iron and so does not interfere with silver's unique properties.) One legendary smith, Ríma, even went so far as to forge double-edged blades, with one edge of elfsteel and the other of chthonic iron, giving its wielder a potent weapon against beings from both Above and Below. These swords (called Bastard Blades, split swords, or Swords of Ríma) were forged in secret to escape the disapproval of Ríma's peers; however, their makership became known, and Ríma was exiled from her people for what were seen as blasphemous bastard creations. Split swords still resurface from time to time, often in the hands of wealthy and ostentatious nobles of the Inner Six or borne by daring adventurers deep into the Earth and far into the Wilds (where, more often than not, the blades are lost once more).

Bronze
"The Deep Dwarves have known the art of working bronze since time immemorial. It is said that they taught it to Ubaal-Kain, great smith of one of the Surdic peoples - but Ubaal broke his somber vows of secrecy and spread the art amongst his tribe. The gravity of this event is not to be underestimated - at that time, iron forging was unknown outside of the Old North, meaning until that point the Surdic peoples made do with tools and weapons of copper and stone, both being poor materials for such applications. With bronze at their disposal, however, the Kain tribe gained enough leverage over their fellow Surds to begin forging a strong coalition of clans, with them at the head. Ubaal-Kain was long dead by this coalition finally grew to become the legendary Kainic Empire; some say he was assassinated at the behest of the Deep Dwarves whose secrecy he spurned, while other tales claim he met his end at the hand of his most promising apprentice by a blade of his own making. In either case, Ubaal-Kain's end was likely violent and untimely, though the legacy of his actions still endures in the ruins of the great cities and fortresses the Kaineans built across their burgeoning empire. In our time, however, the accessibility of iron and the supremacy of steel has rendered bronzemaking a shadow of its former self, just as the Kainic Empire has ceased to cast its claim across the Sands long ago."

Orichalcum
"The metal known as orichalcum was the great secret of the Deep Dwarves. (It is said that it shall forever remain so, as none alive know the art of its making any longer.) It is a metal - likely an alloy rather than a native element of the Earth - with the weight of gold, the color of bronze, and a strength and a sheen far exceeding both metals (and, indeed, ALL known metals). Furthermore, it glows softly when warm - the grasp of a hand soon gives it a soft light, and the taste of warm lifeblood causes it to shine fitfully. Greater heat brings greater light; myths tell of its ancient dwarven smiths drawing it from their mighty furnaces and working it by feel, keeping eyes covered and closed to avoid becoming permanently blinded, as if they wrought and forged a small piece of the holy Sun itself. It is a most envied material: weapons forged from orichalcum are without comparison, entire suits of steel armor are constructed around a single piece of orichalcum platemail recovered from some lost ruin or secret treasure-trove, and jewelry of orichalcum is treasured above all other materials for its unfading durability and the soft glow it gives off when worn against the skin. Myths and folk tales ascribe to it all sorts of magical and mystical properties, though its supreme hardness and its curious luminosity are usually the only characteristics common through all the tales. (Orichalcum's utility against chthonic beings is probably the next most common property ascribed to it, though precious few orichalcum weapons exist to test this hypothesis.)"

Volcanic Glass
"The bile and tears of certain volcanoes often coalesce into a hard smoky glass, which cleaves into preternaturally sharp edges when carefully shattered. Many of the Surdic tribes have known the art of knapping volcanic glass (called "dragonglass" by what oral traditions and later manuscripts have survived to the present) into deadly tips and heads for weapons since before they began recording history. Volcanic glass, despite its noted sharpness, is brittle and chips easily, usually rendering a weapon made of the material useless after a handful of blows. Although common metals have nearly universally replaced dragonglass in all its former uses (save certain styles of jewelry common in the Sands), the Asani have been known since their inception to use concealed blades of wicked-sharp volcanic glass in their ritualized assassinations, a tradition they still reputedly carry on. (It is even said that the Red Jesters have even taken to such weapons; as it is said of the Jesters, "creativity does not require originality.") Folk tales from the Sands ascribe to dragonglass powerful properties against the undead, unclean spirits, and general misfortune, though I am hesitant to grant much credence to such superstition."

Wrought Crystal
"It is unclear whether volcanic glass inspired the invention of wrought crystal; though their usage is similar, the elves of the Old North likely had minimal, if any, indirect contact with the Surds of the Sands. In any case, the secrets of working crystal are no longer known. It was the ancient elves, long before the Northcoming, who used their arts to sing sand into delicate structures of glass. While it seems the majority of wrought crystal is still encountered as jewelry and elaborate statuary of various scales, it seems the elves of the Old North also used it for weapons of supreme sharpness. Armor of such material has not been discovered, however; wrought crystal, though much more durable than mundane glass, still possesses pressure points that cause its careful structure to shatter uselessly, and such weaknesses likely prevented its use in armor or shields. Rather, its martial use seems to have been primarily in the making of spear- and arrow-heads, and occasionally sword or polearm blades. Such specimens as survive have been untarnished by rust and unworn by time, though they perhaps become more prone to shatter suddenly as they age through the millennia. Though such weapons are noted for their sharpness and lightness, perhaps the most surprising property of wrought crystal is that it is magically inert. Never has a spell been found to mar or alter such crystal, nor has any enchantment ever been placed upon an article of it. As such, wrought crystal weapons have gained a reputation as the bane of both Magi and spirits alike, bypassing arcane defenses and cleaving even incorporeal forms as easily as they pierce flesh. There are even tales of chains and cages wrought of crystal for the express purpose of binding spirits and Magi, though I doubt such stories bear any credence. (Perhaps I, a Magus of the Fifth Order, am unwise to write in such cavalier fashion of a possible means to my demise - but who reads my works, anyway? If anyone, it is fellow Magi, likely already wise to such matters. Though I have heard that my writings often inspire anger in my more opinionated readers, I doubt it is enough to drive one to murder - yes?)"

- Quotations excerpted from the writings of Erbius the Lorekeeper, Magus of the Fifth Order


So, you, the Reader, have read through all of that spew - and now you're wondering: "wait, but how can I use this in a game?" Never fear, I'm here. What follows is centered around a 5th Edition D&D ruleset, but should be easy to extrapolate to any system. I'm gonna tell you what should be different about weapons made from various materials.
(Assumed technology level above and below is that wrought iron is the most common material for weapons, but that good steel is also used, but is usually limited to the wealthy or fortunate. If you're running a game with slightly more advanced metallurgy, such that steel is the standard, merely treat steel as the default material and give iron and bronze minor penalties. If you're running a game where steel is unheard-of or a lost technology, simply drop it like it's hot and carry on.)

Silver - vs. fiends and undead, bypasses resistances or immunities to non-magical weapons (i.e. deals full damage). Loses this property if damage dice come up "1" (or, for a simpler system, has a 50% chance of losing this property after an encounter featuring heavy combat).
Iron - vs. celestials and fey, bypasses resistances or immunities to non-magical weapons (i.e. deals full damage). However, bends out of shape on a fumble (natural attack roll of "1"), dealing half damage until professionally repaired.
Steel - +1 to attack and damage. (This is not a magical property. Does not impact monster resistances or vulnerabilities.)
Elfsteel - +1 to attack and damage. Versus fiends and undead, bypasses resistances or immunities to non-magical weapons (i.e. deals full damage).
Bronze - Nothing special.
Orichalcum - +2 to attack and damage. Treat as dealing either its native damage type (piercing/slashing/bludgeoning) or Radiant damage, whichever is better.
Volcanic Glass - +2 to attack and damage. Treat as dealing either its native damage type (piercing/slashing/bludgeoning) or Fire damage, whichever is better (but DOESN'T actually set anything on fire). Shatters if an attack misses an armored or shield-bearing foe.
Wrought Crystal - +2 to attack and damage. Shatters on a fumble (natural attack roll of "1"). Attacks and damage are not affected by any magical effect - so spells like Mage Armor, or a ghost's resistance to non-magical weapons, or armor enchantments are useless defenses. (However, resistances like an Iron Golem's immunity to slashing weapons are still in effect, as that is of mundane origin - iron is hard!)


A bit of background design philosophy:

I'm tired of games like 5E D&D that treat all weapons equally, except magic ones (which, by the way, are seemingly a dime a dozen). I mean, 5E technically has "mithral" and adamantine weapons (I think?), but they're dead boring and never better than just a good ol' +1 sword. I want to bring back a sense of wonder to finding weapons of different materials.

However, I want to avoid the opposite pitfall of games that just spew out a plethora of ten or twenty weapon types, sort them by grade, and just expect players to gradually progress up the list. (TESV Skyrim and Runescape are examples that have figured heavily in my past.) That is boring AND tedious. Different weapon materials shouldn't USUALLY just be "better" than others - there should be situations where each type is good, and no huge differences between the best and worst materials. (Notice above that the weapons only scale from +0 to +2 - and of the three +2 weapons, two of them are susceptible to breakage, and the other is a lost secret, and extremely rare!)


What do you think, Reader? You gonna use or adapt any of this?

There's an XKCD for this, of course.
(From here.)