Nakayama is the most famous of Japan's natural finishing stone mines, due in part to the ancient history surrounding the mine, the marketing skills of the previous owners, and the extremely fine quality of the stones.
Kato-san, who owned the mine from the 1950s to the late 1960s, stamped many of his stones with his own personal ink stamp, while most of the other miners simply wholesaled their products, without any mine stamps or ink stamps, to intermediaries, who stamped them with their registered trademarks, regardless of where they came from.
Kato-san took particular care in the quality of the stones he chose to mark with his seals. He generally did not produce inferior stones, and thus the reputation of his mine was not merely a myth, but built on a solid foundation.
The Nakayama mine was (it closed in 1967) on the east side of Mount Atago, the highest peak in the region just outside of Kyoto. The strata at this site were exceptionally fine, and the stones are known for their uniform grain and grinding speed, which are the most important components of a sharpening finishing stone. This is not to say that all Nakayama stones are perfect and smooth, but most of the authentic older stones that you will find give a special response and excellent sharpening ability. True, they are expensive and not intended for casual users. Most of those sold go to sharpeners, collectors, or professional cabinetmakers or builders.
We have to be careful at this point because many stones sold on e-bay as Nakayama may not be. Genuine Nakayama toishi that Kato-san sold under his own brand may have one or more of these stamps in unused condition. These seals will wear off immediately if the stone is used.
If they don't have any of those stamps, you need to evaluate the seller carefully and clearly. Stones vaguely marked Yamashiro or Honyama are not confirmed by Kato-san as Nakayama or the current owner of the Nakayama mine, Hatanaka-san. It should be noted that Kato-san distributed large quantities of rough stone wholesale to middlemen in Kyoto, who in turn could finish the stones and mark them with their copyrighted stamps, which might include the words Yamashiro or Honyama. Thus, there are Nakayama stones with various ink stamps or none at all.
Of course, over the past 800 years, most of the stones from the Nakayama mine did not have the ink stamps that appeared in the 20th century, so there are unmarked Nakayama stones in Japan. There are trade-used stones with worn stamps, as well as products that were bought from the Nakayama mine as raw material by wholesalers or other miners, or previously mined stones that never had ink stamps. The estates of the Japanese Imperial Household and other private collections must also contain old documented and fine examples. So Yamashita, the gentleman who owns Japan_Tools.com, has said in the past that his father can accurately distinguish stones from specific mines by close inspection. I believe that what he claims is true and it would take years of looking at rocks and talking to miners to get it right. After gaining some knowledge in the field and seeing many authentic examples, one can understand and draw conclusions or assumptions.
Currently, the only new Nakayama stones are released to the market by Hatanaka-san, the current owner of the Nakayama mine. He alone owns and legally uses the name Nakayama Mines, along with the copyrighted names and ink stamps associated with it, even though the mine is physically closed. He reserved the right to ink stamp any inventory left over from generations in his family business, and it is safe to assume that many of his new stones are sourced from the vast stockpiles of raw stone that have been mined since his father was a partner in Kato-san with the rest stone from stock.
Hatanaka-san manages the business as shrewdly as Kato, without flooding the market with stones that move from him through his chosen channels. It also monitors the activities of ink stamp forgers. His company is at the top of the tennen toishi world pyramid, and reputation is the most important thing to him.
From retailers, you will learn that any full (size #30, minimum 205 mm x 75 mm x 24 mm thick), properly sized and documented Nakayama stone for finishing will cost $600 and upwards, and a good stone will cost from $2,000 onwards. But a truly excellent stone (if you can find one on the open market) will cost from $25,000 and upwards. Hatakana-san usually reserves this extremely high quality stone for retail sale to its personal customers. He has stones in his shop with prices as high as USD 100 000.