The following metals are quite rare, and have powerful abilities, regardless of what weapon they are forged into. Most of them are quite difficult to acquire, and once you have it, forging it (or finding someone who knows how) could prove very tricky. This can be an adventure in itself (several, even).
Orichalque: also called Meteoric silver, or Star silver, this metal cannot be naturally found on Norynn. All usable supply of the stuff has been discovered in the starfalls. Once forged, it looks like silver, but with a darker shade in the reflections. Orichalque is light, very light (+2 init, an armour made of it has no Initiative penalty), and very sturdy (it will never bend nor break); It sharpens well, and the edge doesn’t blunt (it always deal +2 damage per damage dice); like gold, it doesn’t oxidise. An Orichalque weapon should cost at least a hundred times the price indicated, and that is if the smith has some! There are more meteorites crashing than we realise, but still, the stuff is very rare. Some say there is a big deposit of the stuff in the lower levels of the Dungeon, under the Boss’s Throne room, but who knows…
Adamant: also called Dead silver, because it is silvery black once forged, and doesn’t reflect images, however polished it is. It reflects light, but in strange ways. Adamant is as heavy as steel, but it will never break nor bend, though. It seems to have a life of its own; a weapon made of Adamant will seem to search for preys, adjusting blows to deal maximum damage (+1 to hit, and +1 damage per dice). Last but not least, Adamant weapon are especially efficient against Undead (nobody knows exactly why) and as such deals them double damage, and criticals on 10, 11 or 12. It also can hurt ethereal beings, dealing them normal damages and normal criticals. Adamant is only found in the outer spheres, the Rock, Coal hell, and Sulphur elementary realm. Some dwarf tribes use complicated rituals to mine the rock for the stuff, but even as such, Adamant is rare. Even when acquired, it can only be melted by elemental fire, and so forging it can prove tricky. It would cost at least a thousand time the indicated price.
Vermilion: The red gold of Faëry, the fabled metal of which faë weapons and armours are forged. It is light and indestructible, as far as humans know, and shines like a dawning sun, a profound red with a golden halo. The only way to acquire such a marvellous weapon is to steal it from a faëric creature, or be offered one as a gift from a faë (very unlikely), as nobody knows where to mine for it (well in the land of Faëry, but where is it?), nor how to forge it (in the cold fire of the Dimensional Firewell, but how can you do that?). All faëry weapons are kind of small (meant to be used by faës, which are small; remember a Faë two handed sword is something like a (very) short sword for a human), but give +2 to init. and each damage dice. It also deals normal damages to faës and can, indeed, take away their last HP.
WHAT ABOUT WOOD?
Metal being the principal material to make weapons nowadays, a number of wooden weapons still exist. Mostly used by semi-savage and barbaric cultures, they can, used properly, prove, if not as deadly as metal ones, deadly enough.
The principal weakness of wood is its relative frailty; each time you roll a critical, you must roll D6. If you get less than the number indicated in the table below (durability), the weapon breaks.
Of course, there is wood, and wood. The following table recap three types of wood, and in order: the price if you buy the weapon, the price if you buy the wood, the durability roll, and the TN to make the weapon.
Type Price(weapon) Price(wood) Dur. Fab.TN
Soft wood (cedar, pine...) ½ max damage 2 5+ 8
Medium wood (willow, chestnut...) max damage 5 4+ 10
Hard wood (oak, maple...) 2x max damage 7 3+ 12
Any blunt weapon may be made of wood without problems. A wooden weapon damage roll is really dependant of its possessor’s strength, so all damages should be read as follow: the base damage is the dice (or dices) indicated, ignoring any “+something”. Then, the hero adds its PHY score. Critical is always “stun”.
Piercing weapons, say arrows, lances, spears may (and indeed are) easily made of wood. The pointy side is sharpened, then burnt to make it resilient. Damages are the same as in the table. Note that an all wooden spear is somewhat lighter than a metal headed one, and so could add +1 to hit and to initiative, but will probably break or blunt if thrown (DM’s discretion). Critical is always “bleed” or “stun” (if the weapon’s blunted).
Slashing weapons made of wood are clearly not a thing. Even if some exist (say a boken, for example) you should treat them as a staff, used with the Specialist: Sword skill.
Note: certain particular woods have specific effects on certain creatures. The most well known are Juniper tree being able to hold faës, and old oak spike paralysing vampires.
Of course there is always a smartass in the group who wants a stone weapon “like in RG Veda”, or a giant bone as a club “it’s so badass”. You may go along the line “what about you just choose in the table?” or you can indulge them.
Stone is heavy, and with a few exceptions (silex and obsidian), not really slashing; plus, it seems hard and resilient, but it is really not when you start pounding it. So, cumbersome and fragile as Bronze, but may (if you like) give a +1 (or +PHY, if you’re in a really good mood) to damage if carved as a blunt weapon.
Bone is nice, but it grows weaker with time, even left alone; so, a bone weapon start as hard wood, and becomes weaker after each durability roll (medium, then soft, then autobreak). It does no special damage, but may cause infections, because of the splinters in the wounds.
YOU WANT A WHAT?
The case I have in mind is the Aztec stone sword, the Macuahuitl: it is made of wood, but the slashing part is obsidian shards, so stone (and of course one of the specials).
You’ll have to improvise; you can treat it as a normal sword, with a durability test; you may want it to cause infection (because obsidian does splinters too); you also may want it to become less and less effective (because of the splinters really) like bone.
As always, it is impossible to cover everything; there will be times of deep questioning indeed. But we covered enough for improvisation to be pretty easy.