I just helped my brother choose a diamond for his girlfriend so I know how confusing it can all be.
It would be much better to get it at a "mom and pop" jeweller because
- Buying from a chainstore you are more of a number than a person. A 'mom and pop' store will go out of business if they don't sell great product with good service.
- Generally chainstores don't have diamonds above I or J colour. You don't want to go under H colour, F or G is ideal (for the average person).
- The cut may not be as good because these are bought by the more serious jewellers. They may tell you that they have perfect cuts (for e.g. hearts and arrows), but it is HIGHLY unlikely, if not impossible. The best cuts go to wholesalers and small jewellery designers.
- A jeweller will be able to give you more advice on the diamond and the setting as separate things. A diamond will stay the same, but often over the years the setting may change.
- Think about the mark-up in the chain stores. You often see the rings at 30-50% off and the product can be pretty low quality.
- Once you have your ring it doesn't matter about whether the store goes out of business or not. It isn't like a toaster that will break. A sotre that is family run is likely to be passed down the family so you still have some assurances.
- The best deals you can get is by going through a diamond wholesaler. They are experts in choosing a diamond which is the key thing you are buying. A jeweller will be able to sell you a diamond cheaper than a chainstore and you will get a superior product.
After you have chosen a diamond you can then go to a jeweller who can work with you to design the setting. The jeweller can create the setting based on the particular diamond you have so that inclusions on the edge can be covered.
Good luck and congratulations.