The <A> of <B> is <C> which is <D>. Whose Attribute is <D>?

1 <A> of <B> = <C>
2 <C> = <D>
---------------------------------
3 <A> of <B> = <D>
---------------------------------
4 <D> = <A> of <B> [<D> is an attribute (<A>) of <B>]

Alternatively,
(<A> of <B>) = <C> = <D>
<D> is not an attribute of <C> it IS <C> !!
<D> is not an attribute of ( the <A> of <B>) it IS (the <A> of <B>)!!
the relationship <of> indicates attribute.

Edit #2 : back on the job after your private explanations.
<quote>
"[The facets](A) of [a diamond or other precious stone](B) are [the flat surfaces](C)
[which have been cut on its outside](D).
I was wondering which one(A/B/C) "which have been cut on its outside"(D) describes/modifies.
Does (D) describes/modifies (B)?
If it does, does it ALWAYS do?( for this pattern: The (A) of (B) is (C) which is (D) )"
<end of quote>

Now that it is clearly a language problem, I need to say that there is no "which IS" in your example, so the initial question is confusing, and does not match what you had in mind. Had it contained "which is", D would have been the attribute of "which", as I previoulsy explained, from a purely grammatical point of view.

Here, D defines (or determines) "the flat surfaces" (C). It is an adjective clause, i.e. a determining (or "essential") relative subordinate, which works as an epithete of C.
You said you were looking for an *attribute*, which only exists if it is introduced by "be" or some other attributive verb. (C) "is" NOT (D) in your example, D is no substitute for "facets" but a determiner or qualifier, so it cannot be an attribute. And I cannot tell what is, or what is not supposed to work in such an unclear pattern.
The only attributive structure here is (A+B) are (C+D)

B but whaaaaaa?