The phrase "importance of which" is correct and can be used in written English.
It is typically used to refer to the significance or relevance of something that has been previously mentioned or will be mentioned in the sentence. Example: The success of a business depends on many factors, one of the most important of which is the quality of its products.
Another important feature is model reproducibility, the importance of which has already been introduced.
One other important element is changing direction, the importance of which, in this regard, has been mostly ignored in the literature so far.
The importance of a domain is expressed relatively to the most important domain 'effectiveness and patient safety', the importance of which is set to 100.
— the importance of which become clear as the outlines of Stevens's true circumstances are revealed.
Or Carole could have found something else in Don Epstein's house, the importance of which only she understood.
Many contend that this patent paved the way for Dr. Tompsett's work, the importance of which is not questioned.
Nearby irrigated farmlands produce large crops of melons, the importance of which is celebrated by an annual festival.
Ultimately, I've found that PGCE students are much more positive because they feel part of a profession, the importance of which cannot be overstated.
The equipment so closely associated with the samurai was the material side of a moral discipline the full importance of which we can no longer appreciate.
Crucially, the community did not cede control of its lands to settlers and managed to retain sovereignty – the issue and importance of which runs throughout the film.
The Fed's recent shift toward more or less explicitly targeting joblessness, instead of just focussing on inflation, is a historic move, the importance of which isn't widely recognized.
