The loss of value when you as a private citizen sell diamonds is going to hurt a lot compared to gold where you only lose a few percentages to premium spread. With shady diamonds you probably lose 80-90% of the value when you attempt to sell them. If I was hiding wealth it would definitely be gold. Maybe just do multiple sites geographically dispersed so if you lose one it’s not such a big deal

or rubies, or emeralds or any other countless gems that are actually rare (but industrially useless...)