Would you rather buy a Banksy NFT Or JPG of a Banksy they are the same image at a fraction of the cost?
Try doing that with a diamond. You walk out of the diamond showroom with it, and its resale value plummets to the value of the base metals, the work of the setting, plus a small fraction for what you had just paid for the diamond.
So the diamond is worth whatever it can be bought / sold for online. If walking out of the store drops it’s value, you’re donating to the store.
Buy an exactly equivalent stone elsewhere if the price drops too much after you buy it.
I wish there was a trustworthy clearinghouse/reseller for used jewelry and gemstones. That would be fantastic.