Consider the following expressions please:

This is my collection, two of which are stolen.

This is my collection, of which two are stolen.

Are these ‘of which’ constructions simply the same as the following:

This is the place, at which I spent most of my time.

This is the place, which I spent most of time at.

Notice that these two above are the same, except for the prepositions’ locations.

The same construction can be used for other prepositions: during which, on which, in which etc.

Does this of which construction mean the same, because if I move the preposition to the end of the sentence, it doesn’t make sense:

This is my collection, of which two are stolen.

This is my collection, which two are stolen of.

So, is this ‘of which’ expression not simply a prepositional phrase? Is it more like an idiom or something?

This is my collection, two of which are stolen.
This is my collection, of which two are stolen.

First of all, you have a singular complement (collection) followed by the plural two. You need to say, “This is my collection of diamonds, two of which are stolen.”

Second, when you have a plural like two, three, some, etc, you need to say “two of which,” “three of which,” “some of which,” etc. You can say, “This is my collection, of which I am proud,” but not when you are using plurals like two, three, and so on.

This is the place, at which I spent most of my time.
This is the place, which I spent most of time at.

This sentence needs to be, “This is the place at which I spent most of my time” or “This is the place where I spent most of my time.” The modifiers are restrictive and do no need commas.

This is my collection, of which two are stolen.
This is my collection, which two are stolen of.

In addition to my corrections above, the of goes with which.
This is my collection of diamonds, two of which are stolen.
This is my collection, of which I am proud.

Your corrections make sense; I just quickly wrote them down, paying little time to make them totally grammatical.

So, are you saying this ‘of which’ construction is the same as all the other prepositional phrases?

This is my collection, of which I am proud.———Is this ‘of which’ different from the other ‘of which’ because the other of which cannot have the preposition at the end like this one can?

This is my collection, which I am proud of.

This is my collection of pearls, two of which are mine.

This one can’t have the prepo at the end. Is this still the same construction?

So Im basically say is preposition PLUS which ever an idiom meaning some else, rather than simply a prepositional phrase that could have the preposition at the end of the sentence?

Thanks again

I am saying that of which comes from the old rules that says you shouldn’t end a sentence with a preposition. In more contemporary English, and especially in spoken English, this rule is broken over and over, simply because not ending the sentence with a preposition often creates a cumbersome sentence.

The most well known story regarding this issue is attributed (incorrectly) to Winston Churchill, who supposedly said, in response to being corrected for breaking the rule, said,

“That is the sort of nonsense up with which I will not put.”

I think you misunderstood me.

I’m saying that it doesn’t make sense at the end of the sentence in this case:

This is my collection, of which I’m proud. This one can have the preposition at the end.

This is my collection of pearls, two of which are not mine. This one CANNOT.

Does that mean it is still a prepositional phrase, or is this an idiom meaning something else?

I think you misunderstood me.

I often misunderstand you.

This is my collection, of which I’m proud. This one can have the preposition at the end.

Yes, as long as you’re will to take the heat from grammar snobs.

This is my collection of pearls, two of which are not mine. This one CANNOT.
Does that mean it is still a prepositional phrase, or is this an idiom meaning something else?

...of which is a prepositional phrase, not an idiom, but it modifies the pronoun two and is bound to it syntactically.

I’ve always wanted to know this.

So,

in which, and of which are always prepositional phrases then??

Because both of these have instances where the preposition cannot exist at the end of the sentence…

And ‘in which’ can mean when (but that doesn’t make it an idiom).

Thanks again. The last few days you have helped me greatly, even if you don’t realise.

Thanks. I enjoy situations that I can help out in (HA HA)

Another question on this:

This is the house, in which the body was found dead.

This is my collection of pearls, two of which are stolen.

These are relative clauses. Why not like a prepositional clauses?

This is the house, in which the body was found dead.

This [house] is the house.
The body was found in the [house].

This is my collection of pearls, two of which are stolen.

This [collection] is my collection of pearls.
Two of [the pearls] are stolen.

The items in bold become the prepositional phases the sentences you wrote. The relative pronouns are just taking the place of the nouns.

p.s. No comma after house in sentence one. ...in which the body was found dead is restrictive and identifies which house.