The other day Howard Schechter from the perfectedge.com generously took the time to give me a very detailed explanation as to what makes a coticule a "Kosher" stone. Howard and I decided that it would be a good idea to share the information with all SRP members in order to clear up a lot of confusion and also prevent some people buying stones under false pretenses. Hopefully a moderator could advise me on whether this should go in the SRP Library or not. Here is the explanation:

Kosher stones. First of all, all belgian stones are kosher in one sense. They
are used by the very discriminating rabbis who slaughter meat according to
Jewish law. The laws are ancient and the type of stone used is critical to the
integrity of the process. One reason it is kosher is because it has never had
any oil on it. If one were to have used oil on it, there is a possibility that
the oil had some kind of fat or oil in it from a non-kosher animal making the
stone "traif" or non-kosher. It could be made kosher again by burying it in the
ground for a year which is a purifying process specified by Jewish law but a
rabbi who kosher slaughters meat for a living probably still wouldn't use it.

That these stones are used by rabbis is the reason they have come to be called
"kosher stones". The other reason has nothing to do with color. The rabbis just
prefer a cream colored stone as that's what they have found to give them
consistent edges on their knives (the knives are also subject to a variety of
laws spelled out in "halachah" which is ancient Jewish law. The belgians (and I)
sell to rabbis who kosher meat and are very familiar with their requirements.
They look for stones which are cream in color and a certain "feel". The cream
they're looking for is not necessarily a pure cream! Jewish law doesn't specify
that and it is just a preference. The feel of the stone is as important. Why?
The knives differ in terms of temper (hardness and softness) and that varies
according to the preferences of the individual rabbi.

The kosher stones I sell are the best of each lot I receive from the belgians.
I have never sold a stone that is pink or speckled to a rabbi as that is not
their preference. When I say "speckles" I mean dots of a different color from
yellow cream. The kosher stones can be different shades of cream in the same
stone.

Now, different colors in a coticule are purely cosmetic and have nothing to do
with the ability of the stone to put a razor sharp edge on steel. In fact, I
have customers who specifically ask me for "figured" coticules because they are
the most beautiful of coticules and the rarest. The figuring can be likened to
the grain in wood but does not indicate areas of hardness or softness. Attached
is a photo of the most beautiful figure I have ever seen on a coticule. The
stone also happens to have that "feel" that a rabbi would look for but probably
wouldn't buy.

I hope this has been helpful. I know that the use of the term "feel" is inexact
and subjective but not everything is quantifiable in this world. Now you know a
lot more about kosher coticules than most other people. Be aware that there is a
lot of misinformation about coticules on the SRP site and some people have a
vested financial interest or other reasons for spreading that kind of
information.

BTW, all of the stones I buy are "Select" grade stones which are the highest
grade stone the belgians sell. They do sell other, lower, grades in Europe.
Terms you hear like "extra choice" or "extra choice selected" come from the
boxes of vintage (older than 50 years) stones and were a marketing gimmick back
in a time when you could pretty much say anything as there was no regulation.

Howard