Question on major or minor magical foci: Beauty - major or minor magical foci.
Without getting too metaphysical, beauty would seem to fall within Imaginum - an enhancement of image or the perception of it. But it would also seem to cross into Corpus/Herbum/Terrum... (e.g., enhancing beauty of a person, painting, sculpture). Leaving mentum asside (as the reaction to the beauty), a major focus on beauty would enhance the targeted item. A minor focus of beauty would embrace the beauty of a particular species - person, painting, music, building, garden,......
The thought is that a magus could use this focus to either enhance the appearance of a thing or the focus could direct the creation (CR) or crafting of a thing (RE) or even transforming something to beauty (MU) (all of course with the proper finesse roll).
I vote for minor as well, since making something more beautiful is, as Fruny pointed out, a matter of finesse rather than spell-level. Thus only Imaginem-spells that make something appear more beautiful will be affected. There are some exeptions for other forms but I think this does not justify it being a major focus
I dont think you need a focus. These spells wouldn't have so high level.
And the focus is not a good solution because you need to invent a lot of 'beautify/modify x' spells.
Consider rather the side effect minor virtue with beauty effect and the same sigil. This way you dont need to invent spells but all of the effects you make will be beautiful.
Also think on the free expression virtue if you want to create beautiful things.