Thread: stone ID

Originally Posted by A_S

Hope this is of at least some help or interest to you.

Extremely so. Thanks !

Interesting...

If the OP is willing to send me a private message
that has some location information I can take an afternoon
and try to see if there is a clue in the local geologic survey library.
I do not need exact info just a general vector from a
town or landmark.

To set expectations that area of the world is complex. It is
possible I could have more questions than answers.

The obvious question can you scratch it with a knife.
Can you scratch it with quartz.
Does it fizz when a drop of dilute acid is applied.

Is there a local university with maps or people that
might answer a question or two. You do not have to
ask about it being a 'hone' just ask about the rock
itself.

The interesting thing about that part of the
world is the complex mineralogy that the metamorphic
terrain generates.

Meta sedimentary rocks could very well produce great hones.
A very fine grained siltstone that was cooked just right
by mother nature could be excellent. If you are happy
to pick and choose you might get a pick-up truck of raw material
that is worth the effort even when a quarry operation would not be justified.

A mud or sand saw made of scrap could trim the blocks to size
trading time for diamonds.

Wow - great posts. It's kind of late, so I'll just make a short reply and be back with better pics and more information tomorrow.

At first, it didn't seem like I could scratch it with a pocket knife, but when I took the tip of an old chef's knife and leaned into it, it did indeed scratch the surface.

I'm going to try to hook up with Superfly here about it. I think he told me once that he knows somebody who's cut hones for him before.

Thanks for all the info!

Originally Posted by northpaw

.....

At first, it didn't seem like I could scratch it with a pocket knife, but when I took the tip of an old chef's knife and leaned into it, it did indeed scratch the surface.

I'm going to try to hook up with Superfly here about it. I think he told me once that he knows somebody who's cut hones for him before.

Thanks for all the info!

Seems to have promise. Get some blocks cut down
at least enough to try. save some chips for later....

I think you are showing the correct edge of the rock for use as a hone.
With an X pattern if the light and dark bits are not exactly the same
hardness it should even out and be consistent over time (as well as good
looking).

No mater what it is, if it hones well it is a hone.

If you want to know exactly what it is, your best bet might be to take it (and a detailed description of where you found it) to the Geology department of the nearest university.

Love to see these homemade hones btw! Thanks for sharing!

Took a little longer to get these pics than I thought. I forgot that it's dark when I leave for work and dark when I get home.

Natural lighting, dry stone. Click to enlarge.







In the first picture, the side that I've honed with is facing towards 10 o'clock.

You can see on the second picture where I started to sand one of the larger sides.

If I'm lucky, I'm getting it cut tomorrow morning.

Snow and an abundance of holidays has prevented me from getting the stone cut yet. Could happen any day, though. I still have hopes for getting a neat little hone out of it.

What we have here is a Chlorite Schist. The green color and the metamorphic texture is a dead givaway. You could call it a Phyllitic Chlorite Schist if it has a sheen to it in the light.

Now, lets talk about relative hardness. Mohs scale of hardness ranks steel well above the Chlorite Group of minerals by quite a bit. Chlorite has a hardness of 2.5(a little harder than soapstone) and steel is 5.5. This is based on pure Chlorite Minerals, but not Chloritic rocks(Rocks are composed of several different minerals). A Chlorite Schist is prodominantly Chlorite, but it is not the only mineral present. Minerals such as quartz, feldspar, muscovite, mica and garnets may be present. Quartz, feldspar and garnets are all harder than steel.

So, what to do? 2 scratch tests. Scratch the top of your thumbnail with the rock to see if is harder than your nail. If you see small scratches, then the Chlortie schist has some inclusions or porphyroblasts, because Chlorite alone is softer than your nails. I suspect you will see some scratches. But, make sure you test many different parts of the rock to make sure it is homogenous.

The next test is to scratch steel. See if it will consistantly scratch the side of your blade or preferrably a kitchen knife.

I cannot stress enough to check the honing surface for large porphyroblasts. These are large minerals such as garnet, quarts, feldspar which are all harder than steel. These will destroy your blade. If you can see them with the naked eye or under a X10 eye piece, they are too big. We are looking for a very fine grained composition.

The obvious next step is to try a blade on it and see what the result is(which I'm sure you have already done). Do not use a razor at first, just grab a chefs knife from the kitchen. Then have a look at the blade road under magnification to see if you are doing any damage.

My gut feeling from looking at the pictures tells me this is going to make a great finishing stone!

Watch out for those porphyroblasts!!!!

P.S. I'm not making this up. I am a Geologist.

Many thanks for the response!

If nothing else, now I know what to call the tiny garnets in a Belgian coticule.

So, are the ideal characteristics for a natural razor finisher the following?

A) basic composition of relatively 'soft' stone

B) porphyroblasts of some sort that are harder than steel but *very* tiny

Also, are the 'active' porphyroblasts in the coveted Japanese natural sharpening stones quartz?

Thanks again!

I would think that the characteristics if a good finishing stone would be one that has a composition of very fine grained(if not microcrystaline) material which is harder than steel, but with a homogenous presence throughout the stone.

I cannot answer to the question about the a
"active" cutting material in a Japanese water stone. Interesting to look into. I do not own one. But will soon enough.

Porphyroblasts only refer to the minerals in a Schist or other metamorphic rocks which is not the prodominant mineral. In your case the prodominant material would be chlorite. If you prove the presence of porphyroblasts which are harder than steel(through the scratch test), but are not present to the naked eye, the only way to identify them is under a polarizing microscope.

This can get very technical, but what is important is grain size and if the material is harder than steel.