I do so wish the black in these sets were as fascinating and multi-hued. Among the more engaging slates are products from Vermont and Virginia that are mottled green/purple/red/black/gray. Now THAT would make some cool go stones!
As I say, these things are eclectic, and I could imagine that if the old Japanese equipment makers had had access to other materials they may not always have made the choices they did.
But beauty is not the only factor - and I'm certain "coolness" would not be a factor at all. Associations, practicality and the local environment are among other characteristics that influenced decisions.
In the case of slate, the variety that goes with hamaguri in the old canon is nachiguro from Wakayama Prefecture - still abundant, I believe. This has very strong associations with ink stones (suzuri), i.e. with calligraphy, and an ancient Japanese gentleman who prided himself on the Four Accomplishments would be familiar with and enjoy this material.
As regards practicality, we have famous examples in the boards being made slightly longer in one direction (for perspective reasons) and black stones being sized differently from white stones to allow for the optical illusion that white and black stones of the same colour would look mismatched. These differences are again part of the old Japanese aesthetic of avoiding pure squares, or pure anything - we see the same thing in calligraphy.
As regards the environment, the light in Japan is if a very bright quality (think Tuscany, perhaps, western as artists did), which makes the size difference between black and white stronger, but is also a factor in the stones. Nachiguro is cool. Clam stones with the stripes on top help deflect the sun's glare. Playing with bright pure white stones and polished slate induces tiredness very quickly in long games. The treatment of the board, and choice of grain, to avoid glare is likewise a factor. There are also environmental factors such as making boards less tasty to insects.
Whenever I see westerners write about their own equipment, or equipment they aspire to, none of these other factors ever seem to cone into it. Prettiness is all. Nothing wrong with that, of course. But personally I esteem, for example, an emotionally mature artist such as Joyce DiDonato over her brilliant but technique-obsessed Juilliard students.
Incidentally, I have never bothered to try to own even a half-decent board and stones. It would be a misfit in my house and in my culture. The only item in this line I have ever had the stirrings of an wish to own is a set of tiger-stripe shogi pieces I saw in Tendo - but likewise only because they are so "pretty", so I do understand the point about purple stones.
But Japanese equipment should be judged within its own framework.
