DO NOT BUY A DIAMOND for any big box jewelry store (I work in this field, as I design and build jewelry as a hobby) The mark up is roughly 300%. They would have you believe that diamonds are rare and precious...nothing is further from the truth they are abundant and quite common
If your serious about buying a really nice stone and don't want to get completely ripped....go to a place like the Diamond Exchange, or the Jewelry Exchange...they have offices nationwide and they save you money buy cutting out any, every, and all middlemen.....but again your still looking at roughly 100% mark-up not to mention diamonds are a scam, pure and simple (money wise). Between the DeBeers diamond cartel, child labor in Third World diamond processing/cutting operations, and blood diamonds/conflict diamonds? used to finance oppression and genocide in Third World African countries, diamonds are just not worth the hassle, guilt, inevitable depreciation, and, let's not forget, thousands upon thousands of questionably spent dollars. (Politics aside)....
...if it were me i would sink my money into Moissanite (looks better than a diamond anyway and runs about $500.00 a carat, or better yet (Tanzanite..at least then you investment would appreciate in value)