He writes a crystalline prose (source)
I find this countable usage of "prose" from the Oxford Dictionaries very unusual. I have never seen "prose" used countably. In contrast, several dictionaries list "prose" as a mass noun and all the examples I have seen in other dictionaries indicate that "prose" is a mass noun, e.g.:
She writes beautiful prose. (source)
A search on Google for "a beautiful prose" has only hundreds of hits and most, if not all, of them are false positives as "a beautiful prose writer/style". Can "prose" really be used uncountably? Is "a beautiful prose" idiomatic? Or is that a mistake in the Oxford Dictionaries?