Q:

"The fact of its being" Grammar

"The fact of its being so rare a flower ought to have made it easier to trace the source of this particular specimen, but in practice it was an impossible task." 

I just found this sentence reading a book and it sounds weird to me. I understand the meaning, but I've never seen or studied this structure in English before. I did some research on the Internet but I could find nothing. Could anyone  explain the grammar? Could you please provide more examples with the same structure?  

A:

That sentence sounds fine to me as an English speaker.
A verb ending in -ing can function as a noun (usually called a 'gerund'). Example: 'His being late inconvenienced us all'
Not the most common way to express it, but grammatically valid nonetheless. 'His' (and 'its' in your example) are adjectives modifying the gerund.
Adding 'the fact of' is a tad redundant but also grammatically okay. It's just a prepositional phrase with the object 'its being'.

so rare a flower

This is a construction in English that takes the form 'So [adjective] a [noun]'. The more colloquial way to express it would be 'such a [adjective] [noun]'.
Neither of these constructions are very common in spoken English, or in plain writing.

