Why buy diamonds online?
Prices are much less online. Diamond dealers online simply don't have anywhere near the overhead stores do. These costs include renting commercial space, staff, security, displays, decor, and especially inventory. These expenses are well factored into the price of your diamonds when buying in a store.

Selection is much better. Stores stock a small amount of items which take a year or more to sell, they don't have the sheer traffic and volume online merchants enjoy. This is why online merchants have a huge stock of readily-customizable products they can turn over quickly. This results in less inventory expenses factored into your jewelry and a much wider selection.