I would like to know which expression is the right one:
they are of high quality OR they are of a high quality.
I thought that of high quality is correct, but surprisingly, both the phrases are in use! Ngram shows '...are of high quality' more frequently used as compared to 'are of a high quality...'
Furthermore, COCA lists only one example with '...are of a high quality' whereas there are 14 for the other phrase.
Personally, I'd prefer using zero article in such instances.