Diamonds are a very poor investment. The dealers are the only ones making money. My MIL has a diamond ring (which she didn’t want in the first place) and when her husband died, the shop where he bought it (very well known here) says they don’t buy diamonds only sell them. Pawn shops are not known for being fair.
Diamonds and really jewelry in general is something you should buy only if you want to enjoy it.....and know up front the idea of resale shouldn’t be a consideration.