Polished Diamonds

The reference for polished diamond prices is the Rapaport Price List; some of the trade work from the IDEX list instead. Either way, round brilliants are priced separately from fancy shapes, and the published number is never the final word — it is the base from which judgement begins.

Once you have graded the stone, read its base price off the list for its weight, colour and clarity. Then, according to the cut, the position of the inclusions and the stone’s other characteristics, raise or lower the figure from that base. A 3-carat F-colour SI2 with a medium cut and inclusions sitting right under the table is not the same diamond — and not the same price — as a 3-carat F-colour SI2 with an excellent cut and a clean table. The list gives them the same starting line; the grader decides where they finish.

Adjusting for cut

Of the 4Cs, cut moves the price most. Any imprecision in the proportions, or carelessness in the finish, symmetry and polish, costs the stone both appearance and brilliance — and the market prices that loss. The premium grid below reads proportions down the side and finish across the top, expressed as a percentage above or below the base price. A dash marks a combination that does not occur.

Proportions \ Finish	Excellent	Very Good	Good	Medium	Poor
Excellent	+10%	+5%	+0%	—	—
Very Good	+5%	+0%	−4%	—	—
Good	+0%	−4%	−10%	−19%	—
Medium	—	—	−21%	−36%	−48%
Poor	—	—	—	−51%	−65%

Adjusting for fluorescence

Fluorescence usually pulls the price down — most sharply in the highest colours and clarities, where any veil is least welcome. The discounts below reflect the negative effect of strong blue fluorescence; a slight reaction costs far less, and by grade P1 & below it scarcely registers at all.

Colour	IF–VVS	VS–SI	P1 & below
D–F	Strong −10% · Slight −5%	Strong −7% · Slight −3%	−0%
G–J	Strong −7% · Slight −3%	Strong −3% · Slight −0%	−0%
K–M	Strong −3% · Slight −0%	−0%	−0%

Yellow fluorescence is treated more harshly still, lowering prices by a further 5–10%.