I was at the movies yesterday and saw a trailer for a Roman(?) movie that showed a scene of a soldier sharpening his sword with a round(ish) rock that he presumable found laying around. This is quite common in these types of movies.
Is it historically accurate? I’m certainly no knife sharpening expert but I remember watching my grandad using a square stone just for this purpose. Having held a lot of knives and rocks, it just doesn’t seem like a found rock would do the job.
I recall an Alton Brown video where he was talking about the difference between “sharpening” and “honing”. Sharpening is when you remove material from the blade to create a new edge, and it is a difficult task best handled by professionals. Honing is when you already have an edge, but it has been bent out of place by use, and you are just “straightening the edge”. I imagine that a sufficiently smooth found rock might have some beneficial effect for that.
FWIW when I am on holiday and in an apartment I like to cook. I can’t rely on the knives to be anywhere near sharp enough but I will use the unglazed underside of a mug as a makeshift whetstone and that does the trick temporarily at least.
It all depends on the hardness of the rock involved.
A “rock” with large amounts of quartz in it, (quartz is Mohs hardness 7 ) will sharpen most metals, or dull them as well.
High quality sharpening stones IIRC grade up about 9 on the hardness scale, being made of synthetic sapphire, (carborundum), but most sharpening stones are a bit softer and thus cheaper.
Sandstone, being rough grained and packed full of quartz, is handy for doing axes and stuff if nothing else is around. Finer grades of sandstone or quartzites etc. just take longer to take away metal.
This is not the normal meaning of honing.
I expect so, but the man had a bunch of foam rubber props showing what he meant. 
Yes, if you’re lucky. First, a sword is quite easy to sharpen. The reason here is that the blade thickness is very small in relation to width, so you have a thinner profile than most knives (less than 7 degrees per side.) All one has to do here is to add a stout sharpening bevel angle at the edge, between 20 degrees and 45 degrees on each side.
If you ever need to sharpen a sword in the field, look for slabs of fine sandstone or silt stone. These are best. Barring that, get a piece of wood, flatten one side, go to a river and apply sand on the flat side. Use progressively finer grit.
You have to pull away from the stone and not cut into it.
Stroke towards you at an angle and away from the stone at the same time.
I’ve used a sidewalk when without anything else in reach.
*im talking rocks and unusual surfaces,
Yes, you can sharpen a sword with a rock, or a cinderblock.
My grandpa taught me how to sharpen an ax on a regular stone. It doesn’t produce as good results as a sharpening stone, but it works quite well when there’s nothing better available.
OK I see it is possible. It always strikes me as contrived scene but, ignorance fought.
Most people who carried an edge for ‘professional’ purposes also had a small pouch with a whetstone in it - that may look like an unassuming hunk of rock. Just because it is not a spiffy manufactured looking item does not mean it isn’t actually carefully crafted.
[and those hand hammer marks everybody loves because it lets you know it was hand hammered? Most of them are put in mechanically to fake you out. A really good finesmith or even blacksmith can leave a wonderfully smooth surface.]