Monthly Diamond Market Update — September 2025

Welcome to our monthly column dedicated to the diamond market. Each month, we analyze key global trends, major events, and the dynamics shaping demand, prices, and trading strategies. Drawing on reliable sources such as Rapaport News, IDEX, and other specialized outlets, we provide an up-to-date, accurate, and trustworthy overview.

Overall Trends

September confirmed an atmosphere of uncertainty and caution in the international diamond market.
De Beers — historically the largest and most influential mining company in the industry, founded in 1888 and synonymous with the global diamond trade for over a century — is currently at the center of a major sale by its parent company, Anglo American. The government of Botswana, a long-standing production partner, has expressed interest in acquiring a majority stake, while other investors, including Angola, are reportedly seeking smaller shares. The outcome of this ongoing transaction could reshape the global balance of the market in the coming months.

Meanwhile, the market remains highly polarized: smaller stones continue to lose ground, while larger ones — particularly long fancy cuts over 2 carats — continue to enjoy strong demand, especially within the high jewelry segment.

Price Trends

According to Rapaport data, the RapNet Diamond Index (RAPI™) for 1-carat round diamonds (color D–H, clarity IF–VS2) recorded a 0.9% decline in September.

Smaller categories showed sharper declines: –6% for 0.30- and 0.50-carat diamonds, while 3-carat stones remained virtually unchanged.

Confirming this trend, the IDEX Polished Price Index — which tracks the average prices of polished diamonds — also shows a negative movement, with sharper declines in the mid-to-small size categories.

The most significant drops were seen in 0.5-carat (–14.8%) and 1-carat (–12.5%) stones, while diamonds between 1.5 and 3 carats experienced more moderate losses. The only exception came from 4-carat diamonds, which recorded a slight increase — further evidence that the market continues to reward larger, higher-quality gems.

Change in polished diamond prices by carat weight, September 2025 vs. September 2024 – Source: IDEX Online Research

Reflections from the World of Diamonds

The “Motswedi”: An African Giant Captivates Europe

Weighing 2,488 carats, the “Motswedi” (meaning “source” in the local Setswana language) is the second-largest diamond ever discovered in the world. Originating from Botswana, it was unveiled in September in Brussels by HB Antwerp, drawing the attention and admiration of museums and international collectors alike.

Together with three sister gems from the same mine, its total estimated value exceeds 100 million USD. Beyond its extraordinary size, the “Motswedi” stands as a symbol of collaboration between Africa and Europe, bringing the themes of ethical sourcing and traceability to the forefront of the high-end diamond market.

A Scientific Discovery That Challenges Chemistry

At the end of September, a team of researchers identified two diamonds from South Africa that contain within them a combination of materials previously thought to be nearly impossible.The gems contain both oxidized carbonates and reduced nickel metal alloys — elements that form under opposite chemical conditions: the former in oxygen-rich environments, the latter in those almost devoid of oxygen.

Normally, these two worlds cannot coexist, as one cancels out the other. The simultaneous presence of both within the same diamond indicates that the stone formed at the boundary between two chemically incompatible zones of the Earth’s mantle, providing the first concrete evidence of the complexity of the processes occurring deep within our planet.

As noted by Scientific American, this discovery not only expands our understanding of how diamonds originate, but also reveals nature’s remarkable ability to create harmony even between forces that appear to be in opposition.