Some old tsubas have TEKKOTSU. As we come close to modern age, a powerful tekkotsu becomes rare.
Now, let's have a look how to make a steel for such tsuba.
 
Recently, one tsuba maker revealed his method for making tekkotsu. He had been studying Owari tsuba for many years. The tsuba made by him have many visible tekkotsu and are a good copy of the Owari style tsuba. His method of creating tsuba is complicated: he puts a lot of small pieces of hard steel between the steel layers during the fold welding work.
We think it is a good method to make a good copy. But I can't see if the Owari tsuba smiths took such a complicated method, for what? It may be not the original way of old Owari tsubas' days.
 
Here is a question: Was tekkotsu designed or it naturally appeared by weathering?
This has created a question for people who evaluate steel tsuba. Sometimes we can find a tsuba with many visible tekkotsu, so it looks to be designed, made intentionally. On the other hand, tekkotsu on some tsubas looks very natural so it seems to come from many years of wearing.
 
Example
     
 
This tsuba has some visible tekkotsu, not only on the rim but also on the very surface. When I got this tsuba, I believed this tekkotsu had naturally appeared by wearing, because this tsuba has a fine radial pattern on both surfaces. If the smith made tekkotsu intentionally, such file work was not necessary. At the time this tsuba was made out, it must have had a plain shape and fine finishing.

I think, some tekkotsu could come into naturally. Then some samurais appreciated it, and then it increased the value of the tsuba. Perhaps this motivates some tsuba makers to create tekkotsu.
But we don't appreciate such designed tekkotsu even if it is very visible. We appreciate a natural feeling.
 
An example of fake tekkotsu
 
The signature is "Nobuie"
This is a product in 19th century copying the Nobuie style in 16th century. But the signature may be put recently. It has very uneven surface on the rim, but it is not tekkotsu. It is designed by the tsuba maker.
The top of the tekkotsu-like rib is not smooth as natural tekkotsu. It doesn't look made by long time wearing. Probably the maker wanted to express a powerful feeling, but it looks too much.

I can think out several methods of the tsuba with natural tekkotsu.
1)
Tsuba is not a blade, therefore the material is not necessary to be as fine as for the blade. Cheaper steel may be enough to make a simple tsuba. One ingot of TAMAHAGANE does not have a uniform quality inside of itself. And in the cheaper tamahagane, the differences are bigger. Rough quality of tamahagane and a few times fold welding, such method may remain the uneven hardness in a tsuba plate.
 
 
2)
In a work shop of a blacksmith for tsuba/armor, there is always a lot of small cut-out pieces that came from tsuba/armor making process. In old days, smiths did not throw them away because iron/steel was very expensive. So they should keep them and used them again as the material, sometimes for tsuba.
Welding such small pieces together, and fold it a few times. It may be good enough for a tsuba, because every piece is already refined by fold welding when it was used to make armors. Such steel can become a fine plate although of uneven quality .
 
 
3)
After 16th century, some iron tsubas were made of three layered steel, a sandwich like construction. Both the surface/outer steels are fine, while the core is not. Such tsuba has a long furrow lined tekkotsu on the rim.