Is there a way to tell the grit coarseness of an unmarked sharpening stone?

With some experience you can tell by running a finger over the surface.

As a general guide:

• grinding stones used for serious repair of knives form damage of long ngelect; will feel disticly rough, not as rough as sand paper but you can definitely feel an abrasive surface.

• sharpening stones : these will have a smooth surface but will have a definite 'drag' or friction to them, a bit like rough paper. These are used for normal sharpening of knives.

• Polishing/finishing stones : these will feel very smooth with just a hint of texture, a bit like magazine pages. Used for refinement and polishing of a cutting edge .

There’s more to it than just grit size and there are complexities that won't show up in your marker test.

What kind of material are they made from? Are they intended as water stones or oil? The waterstone is the hard mineral mixed with weak clay so it comes off and makes a slurry when you work it. On the opposite end are ceramic or metal stones that don’t wear at all but are meant as a permanent surface holding the hard grit (typically diamond).

Of the non-permanent type the amount it’s supposed to wear will vary, and the resulting slurry may be more or less friable, meaning the actual effective grit size changes as you use it!

So, unless they are all the same type meant to be used as a set, you can’t tell anything.

You just have to know the technique appropriate to that kind of stone (oil or water? Work up a slurry or keep it clean?) and get a feel for what that particular stone does for you.

If you don't already know sharpening I think you should get rid of those unknown stones, or ignore them until you do know more. Get instructions and stones to match that have known characteristics. You'll also need secondary materials like little stones used to prime the water stones, strops and compounds to use on them, guides and gauges for getting the angle just right, a way to dress and flatten your stones, stuff to get gunk out of oil stones, etc.

I had a similar issue, oke I am a little seasoned but not very much. Never the less I found for myself a solution that might work for you or others.

First though I will mention my personal issue:

I have about 10 wetstones of different courseness running from 120 to 2000. Some had markings still, some not. Some were knowingly of the same courseness (so twice) Some were of different (coloured) materials (orangie/white) both probably corundum.

The easiest way to compare the coarseness was by listing to the difference in tone of the material grinding over the wetstone (I took an old knive) the finer the grit, the higher the tone I believe. This gave me an easy comparisment and get me sorted out. Of course it does not give you a value! is only compares..
I need to mention that I am pretty tone deaf, so if I can hear the difference anyone should be able too.

The way to do some more determination besides "feeling" with your fingertips might be, to scratch your nail across the surface of the stone. Your fingertips (at least mine) will have difficulty determining if you're actually handling a 2000 grid or 4000 grid...with your nails the difference is easily made...just my 5 pence...

First try cleaning the oil off the stone with some kerosene and a stiff brush. You can soak it for a little while. Then try a flattening technique by taking some wet-dry sandpaper (Gator Power Red Resin 120 or 220 grit works well) on a sheet of glass with some kerosene on it. Rub the stone on the sanding sheet soaked in kerosene in a circular motion to clean and flatten, rinsing with kerosene as you go. I use a small pan to dip the sandpaper in to clean it as I go. After a while it will be clean and flat, exposing the grains of the stone, which need to be exposed anyway to sharpen your tool. With the nice clean stone, use a jeweler's loupe (hand-held lens) of 16x, 20x or 30x to look closely at the grain size. If you could measure the grain size, you could consult a grit-size chart to gauge what you have. But since you will probably not have a way to measure the grain size, you could at least compare it to a stone of known size and get an idea of it. Knowing that natural stones rarely go over 1200 grit, you could at least estimate what you have. If nothing else, you will have learned a lot and had a good time doing it.

Simple suggestion: take the stones to your nearest knife shop, and ask them. That's really the only way that's likely to work.