An 18K ring described as having 4-point and 20.7-point diamonds can be assessed, but the wording does not support one fixed price. Treat “4 points” as 0.04 ct and “20.7 points” as 0.207 ct only after the receipt, tag or report confirms what each number refers to. If both figures are separate diamond weights on the same ring, the conditional total diamond weight is 0.247 ct; that still is not priced like one single 0.247 ct diamond, because a group of small stones usually trades differently from one larger graded stone.

Decode the diamond weights before comparing prices

The GIA diamond carat-weight guide explains that one metric carat is 200 milligrams and each carat is divided into 100 points. Use that conversion to check the description: 4 points = 0.04 ct = 0.008 g, and 20.7 points = 0.207 ct = 0.0414 g. If the document says “total diamond weight,” the number may already include all small diamonds; if it lists “main stone” and “side stones,” the numbers may be separate. Do not add them until the layout is clear.

For the diamond part, weight is only the starting point. The GIA diamond grading report guide describes reports that document diamond identity and quality, including weight, color, clarity and relevant finish or treatment information. Read the fields on the actual report; report services and available grades vary with the stone. If the stones are very small, they may not have individual grading reports, so a buyer may value them as melee or accent stones rather than as independently graded diamonds. If a description calls the diamonds “broken,” ask whether it means small accent stones or actual damage. A chipped or cracked diamond and a loose setting require inspection; the wording alone cannot establish their condition.

Use the correct metal formula

The gold calculation depends on the rate basis. The FTC precious-metal jewelry guide explains that 18K gold is 18 parts gold mixed with six parts other metal. If a buyer quotes a pure-gold price per gram, use:

  • Estimated 18K alloy weight = total ring weight − diamond weight − any non-gold parts the buyer excludes.
  • Pure-gold equivalent = estimated 18K alloy weight × 18 ÷ 24.
  • Metal amount before fees = pure-gold equivalent × buyer’s pure-gold rate per gram.
  • Net metal offer = metal amount before fees − stated testing, refining or service fees.

Calculation example only, using invented rates: suppose a ring weighs 3.20 g, its documents confirm a separate 0.04 ct stone plus 0.207 ct of accents, and all remaining weight is verified 18K alloy. The stones total 0.247 × 0.2 = 0.0494 g. Subtracting them leaves 3.1506 g of alloy, containing a nominal 3.1506 × 18 ÷ 24 = 2.36295 g of pure gold. At an invented rate of 600 yuan per gram of pure gold, that is 1,417.77 yuan before fees. A fee equal to 3% of that amount would leave 1,375.24 yuan, rounded only at the final step. This excludes any separate payment for the diamonds. The extra decimals show the arithmetic, not the precision of a household scale; a real buyer must confirm weight, purity and deductions.

If the buyer instead quotes an 18K-gold buyback rate, the formula changes: estimated 18K alloy weight × the 18K rate − fees. In the same fictional example, an 18K quote of 450 yuan per gram gives 3.1506 × 450 = 1,417.77 yuan before fees—the same metal amount by a different rate basis. Do not multiply by 18/24 again, because that would discount the same purity twice. Keep all rates in the same currency and weight unit. If the quoted rate already includes the buyer’s margin or refining deduction, do not subtract it again. Confirm who keeps any removed stones before authorizing dismantling; do not pry stones out just to weigh the ring.

Decide whether the finished ring adds value

A recycler may pay mainly for metal and very little for small, ungraded diamonds. A second-hand jewelry buyer may pay more if the design, condition, brand, matched stones and documentation help resale. A retail listing is not the same as a completed resale. The practical comparison is to collect three written numbers on the same day: a metal-only quote, a jewelry resale or consignment quote, and any repair/damage deduction. Then compare the net amounts after fees, not the most attractive headline price.

The cleanest way to avoid overpricing is to photograph the ring, weigh it, confirm whether the 0.04 ct and 0.207 ct figures are separate or total, check whether any diamond is chipped, and record whether the buyer’s rate is pure-gold, 18K-gold or finished-jewelry basis. With those facts, the ring can be evaluated consistently. Without them, a precise answer would be guesswork rather than an appraisal.