If you are considering a diamond, buy used. Diamonds don't wear out! If you buy used you can save 80% over retail.
Meet at a public Jewlers, most will let you look at it under a scope and will verify that it is in fact real or fake. Have the used diamond re-mounted in a new custom built *new* ring.
The retail markup on diamonds is just crazy. Don't do it. If you are not familiar with how diamonds are graded by color, clarity and size, you will want to do some research on that first.
My last used diamond purchase was $900 for a 1.66 ct princes cut stone that had a very small chip on the top edge. Had the stone recut down to 1.51 ct for $300 and mounted in a new ring. It apprased for $13,000 but I only have $2,000 investment.
This approach does require a time factor that you may not have.