Given that in the Mystery Cults Verditius section a Minor focus is "Wooden wands", I think that your view may be too restrictive.
I can't see a magus creating wooden wands out of thin air, however enchanting effects into one would work for this type of focus.
Make good backscratchers.
Especially if you've initiated "items of quality" - then they're supernaturally good. 
A Rego Corpus effect?
Pe(Cr)Im
Ahhhhh....
ReHe for those hard to reach places....
Though seriously, being only able to create or manipulate wooden wands would be an almost useless skill.
Don't forget that a Major focus and Minor one work the same, only one has wider scope, it was Errata'd.
PeFu (Perdo Funkum)
You need Intellego - to make it scratch right in that one spot that feels sooooooooo good. 
(Up)
A little return on that...
If, for exemple, a verditius takes a Minor Magical Focus in gold, any enchantment invested in a golden item would benefit from the Focus???
This is disturbing. Logical and cool, yes, but very powerful.
Yup, it is. Makes the expression gold rush get a new dangerous meaning. Rather cool, actually
Xavi
That's the way I read it. That's the nature of the Major Magical Focus. It serves to define a magus. If my focus is gold, I know that all my items (largely) will be constructed from gold. That's quite limiting in itself as I'll probably not be the best swordsmith you're going to find.
A focus in "metal" is broader (still within the legal range) and probably alleviates that problem to a degree, but my enchanted brooms of sweeping aren't going to be as strong as my swords of swishing. And I probably wouldn't enchant an iron broom because, well, because that's just a bit rubbish.
I don't know if I'm really comfortable with this.
This makes Herbal and Terram-based focus way more useful than other focuses, even major ones, especially in the hands of a verditius magi
Agreed. But, the question is, way more useful than what? In my current saga we have a Bonisagus who has an effective magic theory of 9 (I think). My Verditius is up there too. Now, out in the field there's very little the Bonisagus can't do with Ignem. That's way more powerful than my magus. But, inventing devices? That's my speciality. Especially metal ones.
Granted, my focus also applies to casting spells but that tends to be a little less important to me than the lab total.
It's usually the case that a player will pick a direction for their magus and explore that direction as far as possible. In the case of the Bonisagus, he already has frightening levels of Creo Ignem. For me, I get to enchant powerful effects more quickly. Neither of those (yet) have overpowered one character or the other and currently neither has proven to be more useful (I'd put money on it being the focus...) just different.
Swings and roundabouts.