One of my magus' has writing and runes as a magical focus.
I'm interested in enchanting individual runes and words engraved in stone.
Would i simply enchant the correct amount of stone (assuming the rune is carved in stone)?
It seems to me that a rune is an absence of material and as such can't be enchanted. I might be able to enchant the stone block the rune is carved into but not the rune itself. This however isn't terribly mythic or cool.
Any ideas?
A rune is a shape.
An engraving is a depression in a particular shape.
Thus, you enchant the stone with the rune on.
In addition to the above comment, note also that in the strict interpretation, the focus would not apply if simply enchanting runes or pieces of writing. It would only apply if the magical effect being enchanted was about runes or writing.
Thus a magical stylus which can inscribe one's spoken words counts, as does a piece of text which can be understood by anyone*. However, just because the enchantment takes the form of writing does not mean that the focus applies. Note also that there is a (long) discussion on this issue elsewhere on this forum.
Mark
How do you define this focus? Can be used on every magic item he makes if it's inscribed with runes? That would be too wide. In the practice this is equivalent of an 'enchant everything' focus which is not recommended.
If you found a way enchant only a table and join it cleverly to the rock.
There are a couple of threads around here that cover that same Focus ground. There are differing opinions as to whether it's a good thing or not (personally, I don't see any problem with it) but a Rune Focus could indeed be used in some way as an enchant everything focus, always supposing that the magus crafts runes into each thing he uses his focus to enchant.
As with all those kinds of "good for general enchantment" foci, they're usually not much good in spells, so it does tend to even things out a little.
Special Circumstances: Inscribing Runes. And hop! 
marklawford:
I haven't got enough xp playing ArM to judge well the value of the 'enchant everything' focus. I suppose the authors had it when they prohibited this type of focuses. I just wrote in the spirit of the corebook.
I certainly didn't intend to make it a case of
"I enchant PoF in a wand, but i've got runes on the wand so my focus counts"
I'm thinking much more along the lines of
"read the runes engraved upon the doorframe and get a PoF in the face (linked trigger).
So if the runes are engraved on, say, a boulder the size of a house, i'd have to enchant the whole boulder. I couldn't enchant the runes themselves? Thats the way i'm seeing it so far.
I'm seeing the focus more in an academic light since the magus is a real bookworm/intellectual type. I've posted a few items/spells of his on the boards that might give you a feel for his focus. Basically using runes as linked triggers where you have to read them = yes. using runes just to blast things or cast unrelated spells = no.
It's worth searching for those threads as there's the specific example "Swords" from the Mystery Cults book that shows just this kind of focus in the official rules.
Check out those threads, there are good arguments for and against.
Rather than engraving the runes, apply them with ink, liquid gold (MuTe) or another substance. Then the runes are physical objects and could be enchanted (I think).
I would have to say 'No' to this one. Verditius do this with all their items, and it only adds ability scores as a bonus. A focus doubles an Art involved...
-1 Necessary Conditions (runes)
+1 Puissant Magic Theory