It turns out to be radioactive.

It's light, it's beautiful, it's strong.

... but anyone who carries or wears the metal for more than a very short period of time eventually gets sick.

This wasn't known during the initial gold rush and the metal was used to make various things... but eventually the people who handled it most or wore or carried pieces got sick and died leaving behind a town full of the sick families of former prospectors.

Short exposure isn't an issue... but don't even think about carrying chunks in your pack for weeks.

And anyone foolish enough to try to toy with this metal unprepared?

Titanium is actually a fascinating example standing by itself, because it's not that hard to find (it was identified as an element before 1800, and is the ninth-most abundant element in Earth's crust) but it's very difficult to work.

Titanium can't be shaped simply by heating it and pouring or working it like you might do with iron, steel, brass, or the like. The reason is chemical: pure titanium metal melts at 1668 °C (​3034 °F). However, at about 1,200 °C (2,190 °F), it reacts with oxygen to form a variety of oxides. (The most common, titanium dioxide, is a valuable product in its own right; it's a very strong, bright white pigment and has other industrial uses.) If you try working with titanium the same way you would steel, it will literally burn up.

So in order to work with titanium, you need to work it under an inert atmosphere. Normally pure nitrogen is the go-to for industrial uses... but titanium will burn in nitrogen, too. You need an atmosphere made up of a noble gas, like argon. In this time period this might just about have been possible in a lab, on a very small scale, but it's still the better part of a century from industrial applications.

The one caveat is that most cold shaping methods will still work on titanium once it's purified. If you have an ingot of titanium metal, you can stamp it, roll it, carve it, ream it, etc. and it'll work okay. So it might be hard to justify not allowing your players to cut up your miracle door for body armor or the like, but they'll have a very limited amount to work with, and no way to make more.

Absurdly strong metal! Well, in the current location/situation - for example, halfway up a snowy mountain.

Unfortunately, those properties change greatly with temperature - taking our "strong when cold" example, by 20°C it's soft, malleable, and can barely hold its own weight up. By 35°C it's completely melted. A good door for withstanding the elements in the cold, but no security against animals or hot weather.

(For reference: standard "room temperature" is about 18°C-22°C, and the Human Body temperature is typically around the 37°C mark)

It reacts with certain common substances. As an example look at aluminum and mercury: https://youtu.be/IrdYueB9pY4

Imagine this kind of thing happening with something more common, like a certain mixture of water, or an abundant chemical that enemies could easily fetch and use. Your armor might be supa-dupa but if it someone throws a chemical at you and your armor/weapon falls apart or worse kills you... You dont want to be using it.

Edit: as another example of metals reacting with stuff, think Sodium (natrium). It's a metal, it's got weird properties (in this case bad, like being soft enough to cut with a kitchen knife) and it reacts heavily with water. Throw enough in and it'll even explode. "Hold on guys I have to take off my supa-dupa armor and put it in a water-proof bag, it's about to rain" :D

Dont know how to work it yet:

Simply put, they dont know how to use it properly. Pure aluminum is light and strong but sheers easily due to its molecular structure. Add copper to the mix to get construction aluminum and the copper molecules prevent the sheering (if properly cooled), even though copper is counter-intuitively weaker than aluminum it strengthens it! The bits and pieces they found and used were natural deposits of good and bad mixtures. And the only way to work it to the knowledge so far is to chip it into shape, severely limiting what you can make depending on the size of natural material you found. A knife could work, but a full armor...?

Its hard to work it.

As mentioned, Tungsten for example is hard to work. Not only does it form oxides when heating it up, but due to the temperature needed to melt it theres virtually nothing you can heat and contain it in! Its one of the reasons we know much less about Tungsten than other materials. It doesnt need to be that restrictive, but it can be so restrictive that only a handful of top-notch master craftsmen can make something out of it and even then theres a high failure rate. So the limiting factor isnt the material, but the expertise to work it. Who cares if you've got 50 metric tons of the stuff if 10 kilo's is enough for a master craftsman to make several attempts and build one armor and practically no one can afford it? This makes it worthless to most people.

You have answer in your question:

No full armor or bullet proof vest, light canoe, shield etc.
Thing it can be used for : horse thing, knife, small things with no impact from spoon to frypan

The metal is forgeable only in small sizes:

• It may be due to heat (very high temperature) it require to be shaped.


• it proprieties change with size, the more metal you have the more brittle/unstable it become


• Due to it's proprieties it react to much with it's surrounding. A revolver made with this metal don't allow bullets to leave the barrel. A shield reflect bullets but transfer kinetic energy multiplying it by the square size of shield. A canoe/boat/ship is attracted to the nearest large deposit of MGM rather than direction you want the ship to go.

And last thing. The "tool wear". In woodworking there are woods of different hardness. The hard ones are, well, the best but they wear the tools used to shape them. The saw are dull, chisel are made blunt.

Creating things from MGM destroy your tools. Using hammer and anvil on MGM is like using brass hammer on hard steel. The metal is not scarce, like gold, but creating usable things is not price reasonable. The jewellery made from that metal is costly but ONLY after the dozens of tools are destroyed in the process of making in. So the price comes from waste that was required to create MGM "thing" not from the MGM proprieties.

A lot of real-world metals are not intrinsically valuable (like gold is, due to its scarcity), but become valuable in the right context.

"Damascus steel" was once priceless, because it made the best swords, and most metalworkers had no idea how to produce it; but it was made from ordinary iron and carbon, so you wouldn't be rich just because you discovered the ore seams it was made from. The same is true of titanium and alumin(i)um, which can only be worked and/or refined with modern technology.

Conversely, many metals are scarce, but did not become valuable until people discovered uses for them (neodymium magnets, yttrium lasers, tantalum capacitors etc.). In 1870, an ingot of tantalum would certainly be expensive, but if you showed it to anyone their reaction would be "so what?"; today, the trade in tantalum fuels deadly conflict.

A fun idea might be if the MacGuffin metal was some kind of impressive alloy, but it melts at 36°C (perhaps it contains gallium). Getting to and from the town involves two days' travel through Death Valley, and because refrigeration doesn't exist, there's no way to stop the alloy melting on the way.

It's weak to copper*

MGM is great. Relatively abundant. Easy to work in its raw state. But after purified and tempered, it is as light as aluminum and stronger than steel or titanium.

The problem is it is weak to copper. A copper knife will go through it like a hot knife through warm butter. Heck, a copper coin will go through it as easily as a knife. It might be a great metal, but if one can cut through it wil pocket change, it's not terribly useful.

Now from a scientific standpoint it isn't 'weak' to copper. It is highly reactive with copper. If any copper comes in contact with it, it will immediately form Cu₂MGM₂₀, a dodecahedron of MGM atoms around two copper atoms. And the new compound won't react with anything else. It just flakes away as dust, leaving it looking like the copper cut through the MGM. And it's a pain in the neck to split the molecules up again. Since it is a 10 to 1 reaction, one pound of copper will destroy 10 pounds of MGM (assuming they have similar atomic mass), or a copper dagger will only lose a thin layer in cutting through a door, armor, or sword. Gold and silver, also being in the precious metals series, may have a similar or reduced reaction with MGM.

*You could choose a different metal (metal column) instead of copper. Cobalt, nickel, and zinc might work; they are just less likely to be commonly carried (copper being common for coins) and thus discovered as the weakness.

The Titanium answer is a good one, but I'd like to offer some other possibilities: radioactively dangerous to be around.

A real world example is Mag-Thor, which is only slightly radioactive but still needs special handling. If the magic metal is fairly "hot" radioactive, it will cause visible injuries, sickness and death fairly quickly. That discourages people from trying to make bulletproof vests out of it.

The properties aren't that much better

Sure, it's lighter than aluminium and stronger than steel. But it's only 20% lighter and 20% stronger.

The metal has some disadvantages too.

It's got, say, poor fatigue properties. Oh, you didn't notice when it was a door, but it you made a sword out of this you'd be replacing it constantly.

In the 17th century, the British Royal Navy found that ships with lead or copper plating would have iron bolts and nails disintegrate quickly. MGM is similar; if you make armour with an MGM sheet and steel rivets, it's very prone to corrosion.

And it's expensive to produce.

Aluminium is found in an ore called Bauxite which is very common - but expensive amounts of energy are needed to extract the aluminium from the ore, even with modern techniques. Back in 1845, before modern extraction techniques were invented, aluminium cost more than gold.

MGM is the same way. You've found the ore and that the refined it has these good properties, but it's going to cost a lot more than steel because of the amount of, say, potassium you need to extract the metal from the ore.

Difficult to make it consistent, too

For some reason, eight out of ten batches of the metal just don't perform well. And you don't figure that out until right at the end of the manufacturing process, when your armour just isn't as strong as it should be in certain places. Dr MacGuffin thinks the boron used in refining tends to clump together, even though it's the best anyone can make, or possibly that the horse urine was gathered too late in the day. Anyway, the science of the day hasn't been able to solve the problem.

Did I mention it's expensive to process?

If you get MGM red hot in a forge, it reacts with air and gets less strong. It ends up less strong than steel, in fact. For it to keep its strength you have to stop the air getting to it, by processing it in a bath of argon. So you can do it, but your world's existing blacksmiths generally can't work with it. They'd need new equipment which is very expensive.

Oh, and someone else has patented the only extraction process that works

Tycoon Tom has a patent on the only MGM extraction techniques that seem to work well, and he's got big investments in steel mills. He's not really looking to let anyone use his patents. And your setting has very strict intellectual property laws.

Plus it's classified

The government thinks MGM has such important military applications, it's a state secret. All details about it are born secret, even if you discover them without seeing anything secret yourself. The secret applications are mostly in large artillery shell casings, so the properties that make it militarily useful aren't helpful for making swords or anything.

All things considered, its advantages aren't big enough to outweigh the cost and problems.

It never had any value to begin with.
It was a product where the marketing got ahead of the development.
Some smooth talking salesman found himself in possession of the stuff, silver-tongued his way through a huge sale by promising all sorts of valuable properties to the metal, and it caught on. The material started changing hands, shot up in value as a untested commodity because it's so shiny, it has to be great, right?

Large organizations (banks, corporations, government factions) caught wind of the hype and started "Go west, young man" campaigns to get cheap labor in on the ground floor. Promises of wealth and a solid career in the MGM industry brought lots of men and women out to seek their fortune. Soon warehouses all over the map were full of the stuff.

Then the metallurgists got a few samples. People were bringing in nuggets, ingots, anything they could get. All were asking for the miracle devices they were promised to be crafted. Weapons, armor, machine parts, the works.

It turns out it is only useful as an alloy to other metals, usually metals that are so scarce, large-scale practical uses are too far out of scope for the amount of materials any one person or organization can realistically possess. However, the metallic mix is close to 80% MGM and 20% Unobtainium.

Or the process of making the alloy is so time consuming or dangerous to not be worth the effort.

Either way, many small trinkets exist, horseshoes, ashtrays. Whispers exist of a family that has an entire cutlery set made from MGM and Unobtainium (Or just plain MGM if that's what you go for, if so, ignore the next paragraph).

Because of the scarcity of the Unobtainium, MGM values have plummeted to nil. Sure, some people still run mining ventures because the occasional investor can be duped into thinking that when Unobtainium becomes Scarce-but-still-possible-to-obtainium the future of MGM will blow through the roof.

As for now, the abundant surplus of MGM sits uselessly in warehouses, frequently in ghost towns. The value of the material is so low it's not worth paying a few laborers a day's work to move the stock.

The door the party encounters is clearly made of either masterfully crafted MGM or the 80/20 mix.

What if the metal had an inherent, impossible to counteract, and nigh-impossible to slow, decay rate (non-radioactive). When it is in ore form, it is stable and prevented from decay; however, as soon as it is melted down and forged into any form, the decay begins. The larger the item it is forged into, the quicker the onset and severity of the decay, as well as taking a penalty due to the forginig process.

The Decay

This is a base template for the decay. Size differences will change these properties:

• Immediately after forgining it is unchanged, and retains its amazing properties


• After a few weeks, it has lost both durability and some minor bit of its stregth.

As time goes on, it continues the decaying process, which only accelerates with time.

• After three months some twenty percent of its initial strength and durability, therein bringing it closer (but still quite superior) to more attainable, and less questionable, materials


• By the time six months have passed, it is almost identical to the next-best material; the accelerated decay process has now taken over fifty percent of the properties that originally made it special

The Size Penalty

• For small items, the decay rate is as described above. This would include the aforementioned cutlery, horseshoes, hammer and axe heads, and cooking pots/pans


• For medium items, the decay rate is quadrupled (4 times as severe). This would mean that after three months, an item has lost eighty percent of its initial strength and durability due to decay. As well, as soon as the item was completed, it would take a five percent forging penalty. That would put them item at only fifteen percent of its initial state.


• For large items, the decay rate is octupled (8 times as severe). This would mean that before even two months, the item would already be destroyed by the decay alone. As soon as the item was completed, however, it would take an immediate twenty-five percent forging penalty. This would effectively mean that the item would last barely one month. This would include armor, shields, or any wearable item. This would also fit into the game rules that exclude such items from being generally made from MGM

The Forging Requirement

• The metal requires very high skill and a massive amount of heat to forge

While this is likely not a perfect solution, I think it is a servicible one from a pseudo-chemistry standpoint. This would make it so that only the most powerful and rich people would be wealthy enough to even consider using this material. As well, their use would come at great cost to themselves. Considering the difficulty, expense, and relative short-term use of the metal for any practical application, the metal would be untenable for use. While it is easily attained, and anyone could use it, why would they? The combined difficulty of melting down the metal and forming it, along with the decay and forging penalties, makes it something that would be very hard to find any use for overall.

The extraordinary property has no practical use (yet).

Whatever the property is, it'd have to be immediately recognizable, even as an ore. A visual property seems the most likely. People find a metal with, say, a rainbow-colored sheen to it, even while sitting in the ground. Immediately people would assume they've found something special, and try to figure out how to make money off of it.

For a brief period, before any details surface, there's a "gold fever" where people try to grab as much as they can of it. Demand skyrockets, as does the price.

Eventually the metal gets into the hands of scientists, who begin to analyze it. and at the end of the day, they discover that it's essentially oddly-colored aluminum. Some impurities in the ground, combined with trace elements in the water, result in the odd coloring. It doesn't make the metal stronger or more ductile, just...prettier.

Aside from a small market for costume jewelry, some local tourist trinkets, and people who want to show off, there's no benefit to making anything out of the metal. So it's got a small customer base of rich people who don't know what to do with their money, but otherwise, it's not worth the extra trouble. It may only be available in this one vein, but there's a lot of it.

Now, if you want to go long-term, it could be discovered much later that the metal has a higher electrical conductivity, or exhibits odd reactions in a magnetic field, something that makes it useful for certain scientific or industrial implementations many decades later. So suddenly the vein has a use, but people dug quite a bit of it up to make oddly colored horseshoes for the mayor's dressage team, and all of a sudden those items are more valuable for their material than their age as an antique.

That material should be Wolfraam aka tungsten.

Since it can be used for very small stuff, but for large stuff it's hard to use because it's brittle, making it worthless

then maybe add the option of making an alloy of say titanium + tungsten if you want to use it for small/bigger stuff :D

anyway, maybe just adding the option to make stuff out of alloys in your game would make it better, though it might be hard because making an alloy has like at least 3 parameters, the quantity of each metal you use for the alloy, the metal properties, the reactions some metals have on eachother, which can also depend on many other variables like pressure, heat... and from all that you would need to calculate a bunch of variables for the created alloy.

besides that, many alloys haven't really been tested, since there are many alloys possible.

Someone should make an api for this if it doesn't exist already :D, would be nice for an application, even though implementing all real life variables would be like mission impossible.

Anyway, you could keep the variables limited for your game.

It's not waterproof

When it gets wet, it dissolves. This makes it useless for quite a lot of purposes. Armor? Weapons? Hope you never have to fight in the rain... or in the heat where you sweat a lot. Mechanics? Maybe, but only for indoor use. Tableware? Why does my soup taste so metallic and why does my bowl start to leak all of a sudden? Currency? Say goodbye to your wealth when you drop your coin purse into a puddle. Canoe? Maybe you can coat it with waterproof paint, but if the coat gets scratched, you better be a good swimmer. Arts and jewelry? Maybe for an avant-garde artist who wants to make a point about the transience of material possessions and the perishability of beauty.

It doesn't even need to dissolve immediately. If it is susceptible to long-term exposure to moisture then this drawback might not be immediately apparent, but still be enough to make it useless for a lot of purposes.

If you need a scientific explanation for why a metal would be soluble: It might actually be a salt and not a metal. Or it might react chemically with water, like sodium (technically a metal, although it's far too soft for any engineering purpose).

If you wonder how it is possible for a soluble mineral to be found naturally in the ground:

1. 
   rock salt exists


2. It might exist in some form of ore which is waterproof until it gets refined

It got superseded!

(Inspired by / adapted from the answer of @Cadence)

The metal A was immensely valuable because it has good physical properties and is scarce and hard to extract. The metal T, while abundant, was not considered because nobody knew how to work it.

Then people (maybe not everybody!) discovered the trick to work T (cheaply, maybe in nitrogen), and suddenly A is totally worthless because it is much more scarce than T and its physical properties are significantly less interesting.

The metal includes the souls of demons which gives it its useful properties however while this metal is great for almost every use it has one draw back. If you put enough of it together the power in the metal combines and a demon can come out of your item leaving it as a worthless rusted mess of whatever it was

No one wants to form large weapons or armor of this metal as the last thing you need is at an inopportune moment your offense or defense becomes a creature bent on eating your soul. Many once thought that if they only had a shield coated in this metal or a sword that was alloyed with another metal it would be fine, and it was until people found that hitting a partly demon metal shield with a partly demon metal sword pretty reliably destroyed both and summoned an imp

Now as this metal is still great for ax head and knives people still use it every day, but no one wants armor made from it as one poke from the wrong knife and your expensive magic armor is now a demon that is craving human souls

Its so strong, it's not really forgeable, its really heat resistant, so its has no field of use

Edited away:

OPT2:

The market has so much it costs nothing => wothless

If you're going high-fantasy, the metal is useless outside of the region it's found, due to magic. Of note, this doesn't make it completely useless - just impossible to export. So, your shield, bullet proof vest, etc all work - up to a radius of 20 miles from the veins. This could produce a rather distinctive local culture in time, perhaps a seat of government if the metal is useful enough. Projecting force would be difficult, but it could serve effectively as a defensive deterrent.

Oh hey, you're in luck -- there are a bunch of interesting story options to make the metal useless!

As a general theme, a metal is useful if it can be processed into tools and uses where it is more cost-effective (or much more capable, which is really the same thing) than any competing solution. Therefore we are looking to make it very expensive, or have a surprise drawback as a tool (eg. toxicity).

Flaws in MAKING the Tools

We prevent the metal being useful by making it prohibitively expensive or hard to make the tools. One of:

• 
  

  hard to mine the ore

  
  
  

  The ore could be incredibly durable (thus too hard to get in usable amounts), or the mines unstable, or filled with toxic vapours

  
  


• 
  

  hard to refine the ore

  
  
  

  eg. Aluminium. Bauxite ore is hard to get aluminium out of, requiring electrolysis or tedious chemical processing. This is why Napolean's fanciest cutlery was aluminium, and his medium set was gold.

  
  


• 
  

  hard to forge the metal into shapes

  
  
  

  eg. Diamond jewellery didn't really exist in the Roman empire because they couldn't cut the damn things (emeralds preferred). There are similar, boring examples around titanium alloys in modern metallury, or how the industrial revolution was facilitated by new casting techniques that allowed for bigger iron structures.

  
  

Flaws in USING the Tools

You have defined the metal as having some wonderful physical properties, so it is clearly desirable on that basis.

However, there is another option that makes the metal a useless tool -- chemical and biological effects. Therefore you could:

• 
  

  Make it chemically unstable

  
  
  

  The metal rapidly oxidises into a rust that, like iron rust, has a very different crystal structure to the original metal. This means the rust falls off and continually exposes new metal that also rusts (the opposite of aluminium). Thus all tools are prohibitively short-lived

  
  


• 
  

  Make it Toxic I: Poisonous

  
  
  

  Like Mercury or Beryllium, this metal has salts that are toxic and will kill you unpleasantly. Not toxic enough to be immediately obvious, but clear enough that Dr.Macguffin knows and long-term use of the metal is not viable. Bonus points if the metal salt is absorbed through human skin, or causes sterility.

  
  


• 
  

  Make it Toxic II: Radioactive

  
  
  

  Similar to toxicity, there is a wide range of radioactivity that is not immediately obvious, but as users and miners have collapsing immune systems and generally shit themselves to death en-route to systematic organ failure within a few weeks or months, it becomes very clear this metal is BAD NEWS.

  
  

So in conclusion, pick and choose! There are lots of ways that the metal can be made useless, and lots of ways that you can make a very interesting story with those ways :).