Quoted from vid1900:

NEVER buy a diamond. Ever.
If you have ever tried to sell one to a jewelry store, you quickly realize they are completely worthless
You: Hey, I inherited this 1.75 ct diamond. It was appraised by your store for $18,000 a few years ago. What will you give me for it?
Store: That sounds like a fine stone! Bring it in and we'll explain what your options are [they need to get you into the store]
You: [entering store] Hi, here is that diamond I called you about
S: Really nice, good clarity, a prize to your family, no doubt.
Y: Great, what can you give me for it?
S: We can remount it in a man's setting, it would be a great memory of your aunt.
Y: I've been out of work for a year, I really need the cash.
S: Maybe your wife would enjoy it as a renewed vow, a gift of your love?
Y: Honestly, I lost my wife to the pandemic. I just need to cash out.
S: Well sir, we can give you full value credit towards any other jewelry in the store; excluding watches and swords.
Y: I don't wear any jewelry. Seriously, just give me your best cash offer.
S: We don't really buy any jewelry for cash.
Y: So you would not give me $5,000 for this $18,000 stone?
S: I'm sorry.
Y: How about $2,000?
S: It just isn't done. We have 4000 stones in the vault, all on consignment with the broker. We don't pay for any of those until sold.
Y: Can we sell it on consignment?
S: Yes, but be advised, many of the rings in our "Heirloom Collection" have been up for sale for years.
The trick with diamonds, is that all of your older relatives have also discovered that they can't sell them.
So you can just call all your aged aunts and grandmas and ask if they have any suitable diamonds that you can **buy** because you really want a stone that is part of the family.
Of course, your grandma will have a diamond stud where she lost the other half of the pair, or your dead grand aunt's wedding ring. She won't take any money for it, because she is honored that you thought to ask her.
This benefits you in many ways:
1. You don't have to worry that your wife feels you bought a stone that's too small/large.
2. Your wife gets to tell all her friends that it's a family heirloom, and thus inherently it means more than their mall-bought diamonds.
3. Your grandma will be able to tell everyone how proud she was when your wife first put on "her" diamond.
4. The average marriage lasts 8 years, so you know your ex will give you the ring back - she's not a monster who would take away part of your family.
5. That returned ring from your ex will sit in your drawer until your son or nephew starts hinting around that he's getting married - then you can pawn it off on one of them.