A "lowball" offer feels like an insult, but it usually isn't. A jeweler buying your $5,000 retail diamond at $1,500 isn't cheating you — they're running a business that has to make money on the resale.
Here's their real math. They buy at $1,500. They list at retail $5,000 — but they will negotiate, so the actual sale price averages 75-80% of list, or $3,750-$4,000. They hold the stone in a display case for an average 9 months before it sells. They pay rent, salaries, insurance, and lab-test the stone before resale ($75-150 per stone). When they finally sell it, they pay credit-card processing, sales tax overhead, and warranty obligations. Net margin on a $1,500 buy / $3,800 sale: about 30-40%.
That margin sounds high until you account for the diamonds that don't sell. About 1 in 5 stones a jeweler buys turns out to have a flaw they missed, gets damaged in resetting, or simply doesn't move at the price they expected. Those losses come out of the same pool.
The other thing a jeweler is pricing in: you might have lied. Buyers who sell jewelers a stone with a forged certificate, hidden chip, or "recently cleaned" cobbed surface are common. The 25-35% offer assumes some percentage of stones won't actually be what the seller represented.
When the offer IS unfair: when the jeweler refuses to test the stone in front of you, when they insist on a "verbal offer" that drops 20% the moment you bring paperwork, when they tell you GIA or IGI cert grades are wrong without doing their own lab test, or when they offer trade-credit instead of cash. Trade-credit is worth roughly 50¢ on the dollar to you because their store markup is built into the credit value.
Get three written offers. Walk away from the first low one. Most jewelers will price-match an honest competing offer. Always take cash, not trade-credit, unless you were going to spend it there anyway.