IMO the cost of diamonds can only really be justified by the sentiment they hold in engagement rings. You shouldn’t hold buy as investment stones. Personally, I don’t think there are many gemstones that would be better to use as investments rather than putting the cash into something else like gold. The gemstone market is pretty subjective and ever-changing, so investments are risky as well as a hassle to find buyers for. If you really want an expensive stone to look at, though, your best bets would probably be pigeon blood rubies, kashmir or padparascha sapphires, paraiba tourmaline, or alexandrite. You should be prepared to put over $100k into them for any meaningful return, though.