The stones made by the Escher & Sohn company, produced from 1709 to the early 20th century in Germany, have gained an almost legendary status among razor honers. So much so that they, like Kleenex or Coca Cola, have become the name applied to a whole genre of their type, these days simply referred to as Thuringian. Mined in the Thuringia area of central Germany, these metamorphic stones are composed of an abrasive quartz embedded in chalk.
In more recent years the Muller company continues to harvest hones from the same area, but not the same quarries. There have been varying reports on the quality of these with some users having very good performance while at leat one vendor, finding occlusions within a large percentage making them unsuitable for honing razors, discontinued his sales of them.
The stones sold by Timbertools and MST are mined in the Hunsrueck area of Germany, and are said to have a similar composition to the Thuringians, although reports on the stones vary, and not all vendors provide a similary level of quality control. Hunsruecks may cut faster than Thuringians.
The famed Escher stones came in different colours with yellow green, blue green, light green and dark blue being the colours denoted in labels by the Escher company. The colour coding used by Escher to grade their stones may have related to the speed with which they cut. The lighter coloured stones seem to cut faster. The more blue stones may leave a finer edge. Many find there to be little descernable difference with the end result from their differing Eschers. Preference still seems to be a matter of choice/opinion. Some who have the uncommon brown Escher find it to be a finer stone, however it is likely to be from the Vosgienne region, not the Thuringia region.
Other hones made by Escher with different labels were, The Celebrated German Water Razor Hone, Barber's Delight (some labled Yellow/Green), and Barber's Choice. There may have been others that are not known by this writer. Another well known Thuringan hone was the Hohenzollern. These were also found in yellow/green, blue/green and dark blue and are considered on a par with the Escher by the few honers lucky enough to have one of these vintage hones.
Eschers are fine finishing stones. The labeled Eschers suggest using them with a slurry and they were supplied with slurry stones of the same material. Some honers find it effective to begin with a slurry and dilute as they go down to clear water to finish while others use the hone with water only. One way or the other these are wonderful finishing stones for honing razors to a sharp and smooth edge. If there is a consensus on the equivalent grit to a synthetic stone it seems to be in the area of 12 to 15k. Some find an Escher will improve the edge off a Coticule. Some recommend moving as high as 12k before switching to an Escher as they can be slow cutters, but good polishers.