This is where you purchase the diamond separately and then have it set
in a ring before proposing. You will probably make large savings doing
it this way as you will most probably buy the diamond from an online
diamond retailer or independent diamond merchant.
I think it can be a good option as long as you are confident in
your diamond knowledge, know exactly what style of setting your
girlfriend wants, and that you will be meeting her expectations.
This also gives you the option of pulling out of having the diamond
set before the proposal in case you lose your confidence in selecting
the right setting. You then still have the diamond to use to propose to
her, after which you can both shop for the setting.