Both correct?

[Progress]

I would like to ask native speakers of English.

Are both of them correct?
The watch was rather expensive, but it will last a long time.
The watch was rather expensive, which will last a long time.

[Horsa]

The first is correct the second is not.

However if you change the second to -

The watch, which was rather expensive, will last a long time.

you get an acceptable sentence.

[Progress]

Thank you very much, Horsa.