I agree with Kim re the sound. It's just background music so it could automatically play once and that would probably be okay. But it shouldn't keep playing over and over again ... yikes! (Remember that many many people surf the web at work and they do NOT want sounds coming out of their computer unexpectedly! IMO no sound is better for this kind of site ... )
Do switch your text background to something very light (white or very light grey for e.g.) and then make your text darker (dark grey, dark greyish red perhaps, etc ... you'll have to experiment). I have perfect vision and found this extremely difficult to read.
Bring the About Us page (from text nav in footer) into the main nav or sub nav. You want to quickly demonstrate credibility, industry knowledge, etc. Don't hide who you are ... add photos of your consultants too if you can (professional shots). Diamonds are an investment and a luxury so you must treat prospective clients accordingly.
Make the feedback/contact forms as short as possible. Don't ask for too much information which discourages new visitors from making that crucial first connection with you, the seller. (You'll have lots of time to close the deal *after* they call and get to know you ...)
Those are my top of the mind thoughts