In a world naturally formed where some magic exists but is mainly used by sentient species, I'm trying to come up with a more or less scientific way to explain how this special, rare ore (going to be used for fine crafting) could be found in only one place on the continent ?
Generally speaking, there is no way for any ore to be located in just one place on the planet. Ore comes from the center out, so to speak, therough plate tectonics or volcanic action. Consequently, through natural proceses, you will find it elsewhere. An example is diamond mining, which is fairly rare compared to other minerals, yet is mined in a number of locations planetwide. But, what if we stretch reality just enough that it's still believable?
You could go with the theory of meteor, but now you have a limited supply. All the mineral there is came with the meteor, and that might not suit your story. If it can run out, that would be the best solution. If it can't run out, but it's only very diffuclt to obtain, then maybe...
One solution is that the mineral is only obtainable from the inside of a volcanic cone. This would make the mineral plentiful, but quite rare to obtain as there are limits as to how you get at it (mining from the outside in might work...). Better still, push it to underground magma flows, which are so inaccessible that only one is known to exist. The mineral would only exist (without proper preparation) due to the heat or chemical surrounds of magma. Brought to the surface without preparation it quickly oxidizes into something useless.
Old, Really Old
Another solution would be to make it something that needed overwhelming pressure to create, and therefore was pushed up from a great depth very, very long ago. Now you're looking for very high, very old mountains. So high that it might be a problem to breathe just to get at the stuff. There are only a small handful of mountains in the world that would fit this bill. The higher you go, the more you can find... if you can hold your breath long enough.
The material only comes to be when its raw form interacts with cold, pressurized salt water. That would make it accessible only in the deepest parts of the ocean. However, this might be so inaccessible that it renders it useless in your world.
The Elements Came Together
Finally, you could suggest that the raw components of the mineral happend to be pushed up near a magma stream such that they melt and combine creating your final product. While the raw materials are all over the earth, only in this one location did geology happen to reward us with the right combination of materials, heat, and access. Something along the lines of of the Earthblood in Stephen R. Donaldson's The Chronicles of Thomas Covenant, the Unbeliever series.
There are several processes that are rare enough that only one place on Earth can offer its byproducts in viable quantities.
Sodium hexafluoroaluminate, for a while the only flux capable of reliably separating aluminum from the oxidised form, could only be mined from the Ivigtut mine in Greenland; it is possibly the first mineral that has been completely mined out. No more remains (but we've learned to produce it synthetically).
Trinitite is also a very rare artificial "mineral" that was only to be found at Trinity Ground Zero, Alamogordo, New Mexico (or where a nuclear device has been detonated on the ground. Now that I think of it, underground nuclear explosions will also have yielded some other shocktites, and some of them might even re-emerge millennia from now). Not very good for anything except as a novelty, but who knows.
Stishovite is a remarkably resistant silicon oxide that is impervious to fluoridric acid and even resists to dioxigen difluoride, possibly the most hideously aggressive substance known to man (rather than merely be able to burn cold water, it can deflagrate ice on contact). It can only be formed naturally when a meteor strikes the Earth just so.
There are several "shocktites" that are formed in shock metamorphism when the right kind rock is subject to the appropriate kind of meteorite strike and that possess weird properties (mainly unusual piezoelectricity or birefringence).
Fulgurites are more common (as they are produced by lightning discharging on sand). Same considerations apply as for trinitites.
Then there are compounds that are only found (naturally) in not-so-healthy places like the Oklo Gorge, where the geometry of a uranium ore vein allowed the formation of a natural nuclear reactor.
Otherwise, there are could be all sorts of materials created by the not too judicious application of uncontrolled magic; I think I remember the Cathedral knives in Tad Williams' War of the Flowers, made from a magically poisoned glass.
Some magical experiment ran awry could have led to such a devastation that repeating the experiment is out of the question, and yet in the center of the wasteland the very rock could have been transmuted into something exceedingly valuable; and it would make no sense to let it lie there untapped, now would it? Sort of like what is said to have happened to all the valuable medical research that Nazi scientists ran in the death camps; seeing as the victims were already dead, let's at least not let them have died for nothing.
A real-world example
Cryolite (sodium hexafluoroaluminate) is a rare mineral which is essential in the aluminium industry, because it is used as a solvent in the Hall–Héroult process for the electrolysis of aluminium oxide. (See, the magic of making aluminium metal doesn't work without cryolite.)
Small quantities of cryolite have been found in the U.S.A., Canada, Russia, Brazil, the Czech Republic, Namibia, Norway, and Ukraine; but there was only one large deposit on the entire Earth, at Ivigtût in Greenland; the Ivigtût mine was the only cryolite mine in the world.
We mined it all; there are no more commercially viable natural cryolite deposits: cryolite has the dubious distinction of being the only mineral resource mined to commercial extinction. Fortunately it is not hard to make from aluminium oxides, hydrofluoric acid, and sodium hydroxide; all the cryolite used in aluminium smelting nowadays is artificial.
It came from somewhere else, a meteor/comet/ancient spaceship crash site.
Remnant of an older civilization or some historic origin; old industrial/nuclear waste site, the site of the old temple where dark magics were worked, fossilized unicorn bones!
Requires very specific and unlikely geological processes to form; ancient seashells compressed into stones, melted by volcanic fire, put under pressure and then frozen in ice.
Some magical geographical focus points; the ley lines of the world converge to this spot depositing magical energy deforming the natural geology, creating super minerals. The site of an ancient conjunction of the planets, eclipse, or other magical event.
I'm going to go with the oil idea. Millions of years ago, you had an enormous magical forest, full of lightly magical ferns, trees, mosses, and earthworms. Over time, the forest is buried under itself, depositing magical carbon underground, where it is subject to intense heat and pressure. Due to the magical properties of the carbon, that heat and pressure is somewhat stored in the deposit, lending the coal important and impressive properties. When you make coke with this coal, it lends steel and other alloys powerful magic, or even just the ability to contain magic without the use of "Soul Gems" or something.
Unintentional Magical Contamination
On the other hand, perhaps you had a peat bog that was the site of a great and terrible magical battle that lit the skies for miles around, blinding those too close to the location. This baked strange and capricious magic into the peat bog, and any iron harvested from the peat bog can be use to make magical items with random effects.
Or some well regarded priest or hero of the great and eternal GenericGod (blessed be his name) asked nicely for this deposit of tin to be blessed. Now you can forge weapons and engines of war that smite the followers of the terrible BigBad (cursed be those who follow him).
A long, long time ago, right here on our home planet, a race of incredibly sophisticated beings were dropped off during their attempt to hitchhike the known galaxy. One of those beings had a bit of mud with some nanites in it stuck to their boot. These nanite stowaways jumped ship here, burrowing into the ground to find their favorite source of energy, (plutonium, tungsten, or copper). If they love the taste of radioactive plutonium, it would make the ore harder and much more dangerous to harvest. If they love tungsten, then it would take a much more advanced civilization to make use of the "magic", as tungsten has an extremely high melting point. If it was copper, then your civilization would have access to it very early on, as it has a low melting point, and might have a reason to keep using bronze as a primary material far beyond the intended bronze age. It would also give tools made with it a small drawback, as bronze and copper are fairly soft.
Something to consider is that many minerals exist in very small quantities all over the place, but are only concentrated enough for practical extraction in a small number of places.
If the word's extraction and refining technology and/or magic is poor, the low-grade deposits may be completely impossible to exploit (and so one high-concentration mine could be the only source). But if the technology improves and the demand for the mineral is high enough, mining from the lower-grade deposits might become worthwhile.
Allowing a low level of unobtanium production from new mining operations might be a good way to drive conflict between the people in your world. The miners and their customers might need to fight (or flee from) the centralized power that controls the main high-concentration mine when the latter tries to maintain their monopoly. You could also build a whole network of smugglers and "ore launderers" who move and sell the illegally produced goods. Even if this sort of economic conflict isn't what you want your main plot to center on, it might make a good background for your setting.
Another real-world example
Yes, a geological process can be known to happen in one location only. Look up natural nuclear reactors - in a very very few places, uranium has formed a natural reactor. To be exact, only one is known, in an African site.
A natural reactor needed 5 things:
- early earth history (before the proportion of U235 dropped by radioactive decay from 3% to its current 0.7%),
- a uranium rich ore (for concentration),
- an appropriate level of groundwater flow (to moderate the reaction and replace boiled away water),
- open to the atmosphere which was more rich in oxygen than today's (uranium only dissolves in the presence of oxygen),
- remaining at the surface or undisturbed after 2 billion years (because that was the last time these circumstances coexisted on earth, and it it didn't remain at the surface it would never be discoverable or remain intact).
So that sets a precedent for "one or at most very few places" and some ideas for features as well. If your product needs specific features to come about, those might only have existed in one place together.
How else could it happen?
Statistically, although any ore is distributed, the concentration will vary. So you can require a specific concentration range to have existed, and that affects rarity. The geological origin will also matter - did it need to have come from an ore that's had certain history, for example produced from sand that became compressed and then exposed to great heat or pressure? Now you need a location where sand was created at the surface and pulled down at a tectonic plate edge. Again, the specificity can make it rare.
An example: Suppose it needed calcium carbonate from sea animals, with a high proportion of something or other that evolved in a given species. Imagine a species in ancient times had a biology that metabolised and concentrated mercury, and arose in an isolated landlocked sea where it proliferated for a few hundred million years until the sea dried and all such animals became extinct. You now have a thick bed of calcium carbonate and some organic mercury compounds at high levels. Add a few billion years and some handwaved geological circumstances that transformed the newly formed rock to a new form over time, and you have an answer. If you also add heat + pressure, then even if this biology were in multiple locations, the resulting ore might have formed from the sediment deposits in one area only.
Make the place a nature wonder, a bizarre combination of super volcano, freezing snow, hyper heated water and inmortal bacteria create the posibility for this weird and insane organic crystal with magic properties to be created.
Is it really neccessary to have the source being singular?
In any way, there could be mundane explanations:
- The resource has other deposits, but they haven't been found yet (the existing one is a fluke, the others might be much deeper in the crust). For example: diamonds originally were only found in India in relevant amounts.
- There simply is only one major source on that continent (but the material is more common on another one - this can be useful story-wise).
- There is only one major source left on the continent. The other sources are not economically viable or depleted
- Certain materials really depend on the area's conditions and could not form anywhere else (like the different types of amber which depend on prehistoric forests and flooding)
- It could be caused by a meteorite impacting on another material (many minerals are only found in specific areas due to their creation in a single event). This will generally not work for metals, unless we are speaking of meteorite iron.
The existence of magic of course also allows a supernatural source, like a place where a deity once made a big dump ;-).
An answer nobody came up with yet - there used to be lots of _rare and_valuable_element_ on this planet, so for millions of years people have stopped by and mined it. And now there's only place where there is any left.
Which previous visitors didn't find because their sensors were not as modern as mine. Or it's where they parked their spaceships. Or it was buried too deep for them to bother with.
For an actual historical example that almost exactly fits the requirements, consider tin in the Bronze Age. Tin is an essential part of bronze, yet major minable deposits are located far from the Mediterranean center of civilization, in Portugal and Cornwall (which was literally at the edge of the known world).
This Wikipedia article on the tin trade has a link to a map showing one source of tin in both North and South America, none at all in Africa or Australia https://en.wikipedia.org/wiki/Tin_sources_and_trade_in_ancient_times so it is not unrealistic to have only one source of an important ore on a continent.
Perhaps the planet has a magnetic field (a dipole) and over time the ore is pulled to one spot?
Perhaps it is carried by a migrating species, or is bound up in a species that returns to it's home for some specific reason, like spawning.
The volcano idea is good. Perhaps there is a cluster of volcanoes, only in one spot on planet, that release the ore from deep in the planet.
Perhaps something to do with ocean currents and eddies.
I think in the end it is a transport question.
With magic you have the possibility of portals and creatures from other realms.
What if this was the skeleton of an ultra-sized giant (humanoid) that came to the planet a very long time ago and they are actually grave robbing. Of course they don't realize this until they find the skull...
Considering various celestial impacts, like meteors. If you are looking for more material than a singular meteor, maybe a small moon or satellite was for some reason pulled from orbit. That would definitely be catastrophic, but assuming it happened long ago, the remains of the moon could be geographically isolated to maybe a hemisphere, or wherever you want. Maybe the moon or satellite slipped away from a different planet - perhaps it had a very elliptical orbit around its original parent planet, and then at apogee different gravitational fields dominated (some super rare event for the system, maybe "the planets aligned"), pulling it from its stable orbit. That way you could have a large (or small) body of foreign material in a specific geographic region.
All of this would wreak havok on the system's orbits... maybe that could play into your story. What if the freak event of the moon leaving its original orbit and then getting snagged by the other planet, saved the planetoid from its own decaying orbit in the first place.
Although you could have the ore generate in specific geographic conditions, or come from a meteorite, another solution would be to have the sentient species have limited means, knowledge, equipment, or accessibility to the ore. Depending on your plot and world, you can have the ore be just like any other rare material here on earth, except the people in your world have yet to discover more than one source. Or maybe they know there might be more around, but don't have the means to mine it, or get to the ore in the first place, and just got lucky with finding the one source they have, due to some geologic happenstance (like the ore only forms in pockets of the world's strongest material, and your species happened to find a pocket that was open).
I'm trying to come up with a more or less scientific way to explain how this special, rare ore (going to be used for fine crafting) could be found in only one place on the continent ?
You could say that the rare magical ore came from meteorites, and to add complexity to the idea the meteors fell a long time ago when the planet passed through a meteorite belt. One which the planet only passes through every few thousand years.
Our own solar system has such a meteorite belt.
Rather than say the ore can only be found in one place. You could explain that the meteorite is very difficult to find. It has no radiation or electrical interference and looks like a lot of other rocks. The majority of meteorites are small and can't be found on the ground even if you knew where to look.
There is an exception: In one part of the country it is very cold, and ice flows form. The meteorites impact the ice and travel downstream lodged in the ice. When the ice flows reach the bottom of a mountain they melt releasing the meteorite. This creates a river bed where the meteorites can be easily found.
This is kind of how meteorite hunters find stones in the Antarctica.
Another option is to have the mineral be extruded from (directly or after some geologic processing) living organisms, that evolved in one area and can't spread due to surrounding conditions being unfavorable.
Another approach to the "living things" approach...instead of a mineral, could it perhaps be a form of amber from the sap of a type of tree that only grew, in ancient times, in this one area?
What about if this specific mineral is derived from the remains of a very, very old civilization, that underwent some chemical transformation due to pressure and the passage of the eons? I'm basically following the petroleum formation idea, where dead organic matter, through pressure and time, became a valuable oil.
To avoid the problem of it being available everywhere, it could be formed by a very specific combination of materials that were used on an old temple. For example, the specific ratio of iron, carbon, silicon, cobalt and europium (which is used for glow-in-the-dark stuff) could, after many millions of years and under extreme pressure, become a unique ore by itself.