Conceding a point can be expressed by means of
– an adverb (however, nevertheless, etc);
– a preposition + the point in noun form (despite this, in spite of this, despite the fact that, etc);
– a conjunction + the point in the form of a clause (although this is the case, while this is the case, etc).
In Cambridge's First Certificate Trainer, Six Practice Tests, I found the following (p. 87, key p. 208) in a multiple choice exercise about a text entitled 'Fingernails growing faster':
[…] the human fingernail now grows about 3.5 mm a month, compared with just 3 mm seven decades ago. Toenail growth, [gap: A although; B despite; C however; D nevertheless] only about 2 mm per month, was also up on the figure obtained in a similar survey done 70 years ago.
And the answer the key gives is 'A although'. So here, 'although' is used as a kind of preposition! How common is that?