Right you are on the rock -- as an investment, a diamond is for the birds (then again, everything other than AAA-rated general obligation municipal bonds is too these days). De Beers has done an excellent job of brainwashing people into believing that a diamond retains value well. Try selling a used stone and find out how small a fraction of your original purchase price you get (I only know this because I have relatives in the business and they laugh we they hear "a diamond is forever").
And $6,500 is a reasonable amount to spend on a party for friends and family if it doesn't mean incurring long-term debt or compromising important financial goals/obligations like paying off student loans. Remember, it's not even 1 day; it's about 4 hours and has little to do with the rest of your married life.