Random fun. No serious intent as a proposal. However, Planescape fans all know and love Plane Shift, and we're all familiar with the focus required: "A small, forked metal rod. The size and metal type dictates to which plane of existence or alternate dimension the spell sends the affected creatures."
So, random thought. Which metal, which plane, and why?
We've got the following in 3e:
Prime: Iron. The most common metal on most Primes, of course.
Transitive Planes: These, thematically, should be metals appearing so naturally that they can be found and banged into sufficiently precise shape even by pre-Bronze-Age peoples; with sufficient knowledge of the Astral and Ethereal, these plane shifts would allow access to color pools and curtains that could access any world and come back, if you knew the right path. In fact, it might be interesting to make them all variants of iron:
Astral: Meteorite iron.
Ethereal: Thunderbolt iron.
Shadow: Any magnet; natural lodestones could be used. (Less important that this be accessible to early peoples.)
The Inner Planes
Positive: platinum.
Earth: adamantine. Hardness seems symbolically appropriate here.
Water: Cold iron. (Being opposed to fire and all.)
Air: Mithral. (Known for its lightness and associated with elves, much as adamantine is associated with dwarves -- see the SRD equipment for examples.)
Fire: Gold. (However, silver is the fourth of the 'exemplar metals' given special properties in the SRD -- why not group the bunch? Gold could be for Mt. Celestia -- see the additional note there.)
Negative: Bismuth. It's radioactive, so it decays... but only a tiny bit, so it decays verrry, verrry slowly. (Thought about lead, but decided that was more appropriate for the Gray Waste.)
(Let's keep it simple and postulate that any paraelemental and quasielemental planes are alloys of these.)
The Outer Planes
Many of these metals were not isolated until late technological developments, but a fantasy world that knew what it was looking for could probably figure out how to find and isolate them.
Abyss: Chromium. When you want to make it look good on the cheap, what do we do? Chrome it!
Acheron: Tin. For every cheap generalissimo and tin-star sheriff to hit the planes, Acheron is the place.
Arborea: Cobalt, with its lovely deep colors.
Arcadia: Palladium. The name comes from Pallas Athena, a statue of which was supposed to protect Troy; the word itself is a synonym for "something protective." It doesn't tarnish in air... but it does in the presence of moist brimstone (sulfur).
Baator: Thallium. Just on the light side of lead, it is extremely toxic and has been used for poisonings (nicknamed "inheritance powder," according to Wikipedia).
Beastlands: Calcium, the most common metal in most animals, vital for bone, teeth, and mother's milk. Tricky to isolate, but definitely associated to animalkind.
Bytopia: Nickel. Besides being a money pun for Tradegate, nickel's a good hard worker that's resistant to rust and most often used with other metals in alloys.
Carceri: Copper. Seems to fit the air of this plane perfectly. Besides being an English pun (it is a prison after all), it mocks the values of the thieves and lawbreakers which end up here.
Elysium: Aluminium. Nontoxic, light and unburdensome. Perfect for the plane of peaceful rest.
Gehenna: Magnesium. Ready to blow at a spark!
Gray Waste: lead. Thunk. Heavy. Gray Poisonous. Stable. Indeed, leaden is a word often used to describe the Waste....
Limbo: Gallium. Cast fast -- gallium melts in the hand!
Mechanus: steel, a necessarily worked metal.
Mt. Celestia: Silver, with its mythic overtones of justice and purity. (Note that the layers themselves have metallic associations. Also, see the note under Fire; you could just as easily make Mt. Celestia gold, and put silver with Fire, to group the "four exemplar metals" found in the SRD 'special materials' as an elemental arrangement, with darkwood as the 'shadowy, neutral center' of the materials.)
Outlands: Iridium. Stubborn but aloof, it is incredibly nonreactive, extremely difficult to work, and possibly the densest natural element.
Pandemonium: Lithium. (The outside of which tarnishes black in oxygen; naturally, a fantasy world probably doesn't know much about its medicinal use.)
Ysgard: Tungsten. Hardens steel, high melting point (good when you're sitting above Muspelheim), and is resistant to corrosion. That's the honor of Ysgard for you.
Hmmm. An interesting thought. Personally I'd make the inner planes' metals more physically blunt (Pure metals and such) and the outer planes' metals more related to the meaning or thought behind them. This would relate better to the nature of each group's very makeup. Off the top of my head, I'd do it something like this:
Prime: Different for each particular sphere. Usually a common metal, so the rod for Athas would definitely not be iron, and so on.
Earth: Iron
Air: Silver
Fire: Gold
Water: Nickel
Positive: Platinum
Negative: Lead
I agree with the idea of alloys for the para-elements, but for the quasi-elementals I'd do something different. Negative quasi-elemental planes would have rods of a rusted or tarnished version of the elemental plane's key (Yes, I know that gold doesn't tarnish or rust, but if you want to get to Ash you'll just have to make it happen). Positive-paraelementals would require the same rod as the elemental plane, but purged of all impurities in an absolutely pure form.
Shadow: A rod of pure steel must be completely destroyed by the caster. Broken, then melted, then ground to dust, then allowed to rust away, for instance. The memory of that rod (it's immaterial shadow) is all that is required for the spell
Astral: Astral Driftmetal (It can only be found in the Astral, but if planar lore is to be believed, the Astral was never really intended to be visited anyway)
Ethereal: A rod of silver enchanted so that half of the rod is already shunted into the Ethereal
Mechanus: The metal used is irrelevant, but its dimensions must be precise
Arcadia: A copper rod made by an expert smith
Mount Celestia: A mithril rod blessed by seven different faiths
Bytopia: One tine of the rod is iron, the other is steel to reflect the civilized and uncivilized aspects of the plane
Elysium: A simple rod of bronze made by a being that is perfectly content with its place in life
Beastlands: A lead rod crudely shaped without the use of any tools (although mashing it into shape with a rock is acceptible)
Arborea: An artfully spiraled rod of silver and gold
Ysgard: A rod forged from a broken shield and covered with runes of strength
Limbo: A hollow glass container in the shape of the required rod 3/4 filled with mercury
Pandemonium: A twisted tuning fork of bronze that creates a different tone every time it is struck
Abyss: An amalgum of at least five different metals smelted together and quenched in blood
Carceri: A roughly cast rod formed of melted keys, locks, or bars of any metal
Grey Wastes: A silver rod buried in a graveyard until it has tarnished to a dull grey
Gehenna: An iron rod heated to a dull red. The spell fails if the rod isn't hot enough to burn flesh
Baator: A Green steel rod made from nine pieces welded together
Acheron: A Cold iron rod with only right angles
Outlands: A titanium rod forged in an area with no magic, and quenched in an area with magic (Can be accomplished near the edge of the very first ring in the outlands, or elsewhere with liberal applications of an Antimagic Sphere spell)
Demiplanes: Different for each plane, of course, but the rods for many created demiplanes incorporate a symbol related to its creator.
Far Realms: The metal required for this key cannot exist outside of the Far Realms, and thus it is never a valid target for the spell
Hmmm. Looking back, that took a lot longer than I wanted to spend on it. Heh. Oh well. I'd be interested to see other people's ideas, too. Great idea for a topic, Jem