"You wrote such a nice email explaing the situation, and the fact you are a GIA specialist suggests to me I can rely on you..."
"WOW! I had to stop and think about this for awhile to remember having written you. Thank you so much for following up with me.
However, I no longer have interest in said gemstone. But I am blown away by your customer service and will definitely keep that in mind for when I am shopping again."