I have the $130 hone that you refer to in the advertisment.

It comes from this man in Germany.

MST - Müller Schleiftechnik

You need to select "Neue Produkte" on the left of the home page.

MST however only sell to agents and not direct to the public.

First let me say it is the best finishing hone I have ever used. It imparts a sharper edge than a yellow coticule and a much smoother edge. It works in and around the 10,000 to 14,000 grit level whilst a yellow coticule in my experience works in and around the 6000 to 10000 grit level, depending on with or without slurry etc.,

I totally disagree with the agents comments about not using it to follow a coticule.
This is exactly what I do. I bought it and tried it after using my coticue and was blown away with just how good it is. After you have finished with the coticule, the slurry on the Escher turns dark with the extra polishing this hone imparts. The hone with just water adds another level of polish.

I don't know what you mean by old eschers. This stone has been in the ground for centuries, how old do you want it to be. Just because something was mined fifty years ago doesn't make it better. Why should it?

As to color, it sounds from the descriptions on the internet sites that it varies from green to blue gray.
Mine, the larger hone is lightish green and the slurry is creamy green. It is softer than a coticule and a very dense material. It gives great feedback in use.

Anyway, you now know from somebody who actually owns and uses one that it is fantastic. It is also very reasonable in terms of price. They won't be around long it would appear if you believe what MST say on their web site. Anyway, mine is a keeper as they say. It doesn't however replace the yellow Coticule, it takes off from where the coticule finishes.

The other thing, is that many are truly satisfied with a coticule followed by chromium oxide and then a strop. In fact I find that process to be perfectly adequate. So you could say that the escher is totally unnecessary. For me however it is another level of refinement before hitting the chromium oxide. But boy the edges are a shiny mirror finish and feel no sharper but oh so smooth.