Polished-Diamond Price Index
Polished-Diamond Price Index
The benchmark indices that track wholesale prices in the polished diamond trade
A polished-diamond price index is a published benchmark tracking wholesale prices for polished diamonds across standardised size, colour, clarity, and shape categories. The principal indices in the trade are the Rapaport Diamond Report, the IDEX Online price index, and the Polished Prices benchmark, with additional regional and specialised indices serving particular niches. These indices aggregate transaction data into reference price tables that the trade uses for pricing guidance, inventory valuation, and contract negotiation.
Why the trade needs an index
Diamonds are not a homogeneous commodity. Each polished stone has a unique combination of carat weight, colour, clarity, cut grade, polish, symmetry, fluorescence, and proportions, and there is no terminal market in the sense that gold or copper has. Pricing therefore needs reference points organised by category, and an index supplies these by aggregating transaction data from dealers, bourses, and wholesale buyers and publishing the results in a tabular format that any participant can read.
Without indices, each transaction would require independent price discovery, with each side relying on its own inventory and contacts. The index format collapses this into a starting point: dealers negotiate against the published price, with discounts or premiums applied for specific stone characteristics that the index aggregation does not capture.
The principal indices
The Rapaport Diamond Report, founded by Martin Rapaport in 1978 and published weekly out of New York, was the first widely accepted polished-diamond price benchmark and remains the most influential. The Rap List structures prices by size, colour, and clarity in cents per carat for round-brilliant diamonds, with separate sheets for fancy shapes. Discounts to the Rap List are the standard format for trade quotes; a stone might be quoted at minus eighteen percent or minus twenty-four percent off the relevant Rap.
The IDEX Online price index, launched in the early 2000s, draws on transaction data submitted by member dealers and trades on the IDEX online platform. IDEX prices are published as an alternative to Rapaport and are sometimes referenced in negotiations where the parties have not agreed on the Rap as the benchmark.
The Polished Prices benchmark, operated by Polished Prices Ltd in London, offers a third independent set of price tables drawn from transaction data and dealer surveys. Polished Prices serves clients seeking an alternative reference and is used by some banks and investors who value its independent provenance.
Methodology and limitations
All polished-diamond indices share a common limitation: the underlying market is fragmented and partially opaque. Transaction prices are not all reported, the data set is incomplete, and the categories used by the index do not capture every variable that affects an individual stone's price. The index is a starting point, not a definitive price; experienced dealers know the categories where the index runs above or below actual transaction levels and adjust accordingly.
The differences between indices reflect different methodological choices: which categories to publish, how to weight transaction data, how often to update, and how to handle thin markets in particular size and quality combinations. These differences mean that the same stone may be assigned slightly different reference prices by different indices, and the choice of index in a contract can be commercially significant.
Use in the trade
Diamond dealers, manufacturers, retailers, banks, and investors all use the indices in different ways. Dealers reference them as the basis for buy and sell quotes. Manufacturers use them to value inventory for accounting and bank-financing purposes. Retailers use them as a sanity check against supplier quotes. Banks use them as collateral valuation references for diamond-stock financing. Investors use them as performance benchmarks for diamond investment vehicles, although the lack of a transparent terminal market means the indices are reference tools rather than tradeable instruments in the sense that a futures-traded commodity index is.