There are 2 main types of layers in the Kyoto mines, and each type contains several varieties of stone. The Tomae type has dozens of individual layers. The Suita type has several different individual layers. Within those layers are varieties of stone which are specific to the type. For instance the Tomae type has layers of kiita, asagi, karasu, goshu, iro, namito, asia and so forth. The Suita type has layers of those named like tenjyou, hon, shiki, shiro and so forth. So there’s no renge in tomae, and no kiita within the suita layers.
Asagi is one of the more common Tomae layers. Asa means pale, Gi (same as Ki in kiita) means yellow, so literally asagi means light or pale yellow. So within the mine systems at different depths you will find asagi with more or less Gi. It is the clays or binder that possess the color which adds color casts to the stone, the cutting silica grit particles in the pure form are essentially clear or colorless. Therefore richer the percentage of clays, the darker or more colorful the stones and these are usually somewhat softer, and by default the lighter or colorless stones including gray are usually the hardest (less % of clay binders). The greenish cast to some asagi stones, those being only moderately hard is the result of the optical blending of various color binders, in this case mostly yellow, and yellow optically mixed with gray/blue blends and is read to our brain through our eyes as green. The ulta hard gray/blue in higher silica concentrated stones and their low binder density content make up a stone that is somewhat translucent. Translucent stones more easily reflects ambient white light closer to its original white form from microns deeper within the stone surface. This reflected white light as it passes back towards our eye is tainted by the presence of opaque colored clay particles and the light is mixed and appears to have noticeably bluish cast. In kiita stones the same occurs and you can read different shades of yellow that all bespeak of the stones binder/silica content. Yellowiest is not necessarily better, but certain steel react differently to higher or lower mixes of silica and binders.
Harder of softer is a relative term but most will agree that a very light gray Tomae stone like the one in the photos is harder than the various orange or yellow kiita stones. The same with the Suita layers, the shallower layers tend to be more colorful while the lowest or deepest layers like the Shiro-suita (white-suita) are markedly harder.
Habutae is a pure white suita stone that is named after the purest whitest silk used as lining in womens kimono or is also found in a brides kimono.
Renge is only found in suita stones (not in tomae) and it can be the typical red or even black with purple in-between. Renge is a concentration of ancient radiolarian that were present at the time the stones were formed.
Nashiji (pear skin) or fine light brown (or white on darker stones) is found only in tomae stones.
Yaki is a concentration of alternate minerals and can be found in both suita and tomae stones. It is normally considered a detractor, the darker the more toxic.
The stones structures that make up the Kyoto Yamashiro mines are what’s called MetaSedimentary stone, sedimentary stone that has been altered in one way or another after the initial formation, it has been metamorphosis by heat or additional pressure, etc..
The really deep stones in the mine structure have been morphed by additional pressure from the sheer weight of the mountain above, with the result that many of the clays were squeezed out. The color providing clays are expelled leaving voids between silica particles which are force and almost welded together thus leaving very hard light color gray or colorless silica white stones. These very hard stones are the most plentiful stones in the mine layers, and are plentiful (and cheap) in the stone industry inventory because they were always considered too hard and scratchy to use.
The stones in the shallower layers were morphed over time by being exposed to a closer source of oxygen, free flowing soluble minerals in ground water, and all stones have been subjected to volcanic heat to greater or lesser degrees. A few examples.
You can see that these few above mentioned variables indeed with additional outside forces potentially created 1000s of unique and different stone characteristics in the 60 or so separate strata (iota) layer just within one (75-90ft thick) mine structure for instance lets say Ozuku. Concurrently, if you take into consideration the geographical location of the current location (Japan) of the mines in the Yamashiro-Umegahata region weighed against the original birth location of the sedimentation event that was not far from the Hawaiian Islands millions of years ago, then you can see that the individulal characteristics (although subtle) of the stones from the dozens of mines location between Narutaki to the east and Ohira to the west, a distance of about 15 miles it is fathomable that those unique characteristics can multiply into the millions if you could logically identify and catalog them.
The miners usually only are concerned with their own mines inventory, the wholesalers are only concerned as to how they can move their product in volume, the retailers these days are mainly concerned with next months rent. For the average straight razor or tool user in the U.S. a lot of this doesn’t matter, most guys are looking for a deal based looks and price, but looks can provide an insight into how a stone might perform under the steel while propelled by our digits. Japan is full of passionate hobbyists’ with scientifically orientated attitudes and work habits, too bad for us that their really interesting with original source and first hand information blogs, books, pamphlets and studies are so obscure and will never be translated into English. The meat and bones of the natural stone awasetoishi world is so apparently esoteric for the modern man it boarders on the, well you fill in the words.