I’ve only owned one investment quality stone in my life time. It was described by the appraiser as I think, a brilliant fancy since it had an extra facet. It was about 1 carat. We indexed it. It was stolen. We got the money from the insurance company. Used the money for more important stuff. I do miss it but hey. Not too much.
I use to sell jewelry. You never buy diamonds retail as an investment piece. They are like cars. As soon as you leave the store the price starts ticking downwards.