This is a natural stone collected near Azenotani in Sennan City, Osaka Prefecture, and confirmed through test sharpening to be suitable for use as a whetstone.

Azenotani is a valley in Sennan City that is also known as an access point to Mount Okiku. Geologically, the surrounding area contains the Azenotani Formation, also known as the Azenotani Mudstone Member, of the Izumi Group. This formation dates from the Maastrichtian stage of the Late Cretaceous period, approximately 70.6 to 66 million years ago.

The Azenotani Formation is composed primarily of fine-grained silty mudstone. When their particle size and bonding structure are suitable, sedimentary rocks of this type can function effectively as natural whetstones. However, “Azenotani-to” is not a historically established whetstone name, and no records have been found indicating that whetstones were commercially quarried in this area. This specimen was collected near Azenotani and tested to confirm its suitability as a sharpening stone.

The stone has a hardness of approximately Lv. 4.5+ on a five-level scale. Its surface is considerably harder than that of a typical medium-grit stone and remains highly stable during use, allowing precise sharpening while maintaining the flatness of the blade surface.

During test sharpening, the stone did not naturally produce a large amount of slurry. Instead, it gave the impression that abrasive particles firmly fixed within its hard, dense surface were directly cutting the steel. Its sharpening action therefore appears to rely more on fixed abrasives than on slurry.

Rather than creating a soft, hazy finish on the cladding, it tends to refine the existing scratches while producing a brighter, tighter luster. Its character differs from softer medium-grit stones that generate abundant slurry, and it is particularly effective for sharpening the edge and maintaining a precise, flat bevel.

It is especially suitable for woodworking tools requiring a high degree of surface flatness, including planes, chisels, kiridashi knives, utility knives, and carving tools.

On traditional Japanese knives such as Deba, Yanagiba, and Usuba, it can be used to adjust the steel edge and refine scratches left by a coarser medium-grit stone. It may also be used on high-hardness kitchen knives to refine the edge geometry after the coarse-sharpening stage and prepare the blade for the next finishing step. This makes it an interesting intermediate finisher or pre-finisher before moving on to a finer natural finishing stone.

However, this is not a particularly aggressive stone. It is therefore unsuitable for repairing large chips, substantially changing the edge geometry, or quickly removing deep scratches left by a coarse stone. Its qualities are best utilized after the foundation has been properly prepared with a coarse stone or a more abrasive synthetic medium-grit stone.

As the stone produces relatively little slurry on its own, the use of a diamond plate or Nagura is recommended when necessary. Preparing slurry beforehand improves the initial sharpening response and provides a gentler feel against the blade. It can also help reduce the brightness of the finish on the soft cladding.

On the other hand, this stone may not be ideal for knives with uneven wide bevels, soft stainless cladding, or applications requiring a uniform, soft kasumi finish created with abundant slurry. Because an extremely hard stone selectively contacts the higher areas of the blade, an insufficiently flattened bevel may develop isolated polished areas and an uneven or patchy appearance.

When finishing a wide bevel, we recommend flattening and preparing the bevel thoroughly, generating slurry beforehand, and sharpening with light pressure.

Originating from the fine-grained silty mudstone of the Late Cretaceous Azenotani Formation, this exceptionally hard natural medium-grit stone offers excellent surface stability and a distinctive ability to produce a bright, tightly refined finish on soft cladding. It is recommended for users who value precise edge formation and flat-surface sharpening rather than the softer, slurry-rich action of conventional medium-grit stones.