Hi Robert and all,
there are indeed sources on the Internet that state, that Tekkotsu might be formed by wearing the Tsuba close to thr Kimono (see this oner for example: http://www.ksky.ne.jp/~sumie99/tekkotsu.html ).
In my opinion these are "impurities" emerged during the forging process. But of course not all Tsuba show Tekkotsu as thie depends on the material and the specific kind of forging.
Iron bones had a certain attraction and are related to the "tea taste" that developed in the 16th Century emphasizing simplicity, rusticness and other humble qualities in the tea ceremony. These aethetic principles were incorporated in many other parts of Japanese art and handcrafts.
For some Tsuba schools those bones are Kantei (attribution) points.
There are also Tsuba where the Tekkotsu has been simply filed off as they did not meet the aesthetic demands of the artist.
Just my quick thoughts....