I think, some tekkotsu could come into naturally. Then
some samurais appreciated it so much as it increased the value of the tsuba.
Perhaps this motivates some tsuba makers to create tekkotsu.
But we don't appreciate such purposed tekkotsu as well as naturally coming 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 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 worn out.
Probably the maker wanted to express a powerful feeling, but the result smells purposeful.
I can think out several methods of the tsuba with natural tekkotsu.
Tsuba is not a blade, therefore the material is not necessary to be as fine as for the blade.
The cheaper steel may be enough to make a simple tsuba.
One block 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 will create a tsuba plate with irregular hardness inside.
In a work shop of a blacksmith for tsuba/armour, there is always a lot of small cut-out pieces that came from tsuba/armour making process.
In old days the smiths did not throw it away because iron/steel was very expensive.
So they stored it and used it again as the material, sometimes for a tsuba.
To weld such small pieces together, and to 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 armours.
Such steel can become a fine plate although of uneven quality .
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.