A geological anomaly
The Argyle diamond deposit is not a typical mine. Its source is the AK1 pipe, a volcanic structure known as a diatreme, which formed roughly 1.2 to 1.3 billion years ago. Unlike most of the world's diamond mines, which are situated in ancient continental cores called cratons and exploit volcanic rock called kimberlite, the Argyle pipe is made of a rarer rock: olivine lamproite. It is the first commercially successful diamond mine located on a lamproite pipe. The pipe itself is a long, narrow feature, stretching about 1,600 meters in length and varying from 150 to 600 meters in width.
The formation of the pipe was a violent event. Lamproite magma, originating deep within the Earth's mantle, erupted explosively through a zone of weakness in the crust. This eruption carried diamonds, which had formed even earlier under immense pressure, to the surface. The diamonds themselves have been dated to about 1.58 billion years old. The pipe's structure suggests it was formed by multiple phreatomagmatic eruptions, where the hot magma interacted explosively with groundwater. This process created a complex mixture of volcanic tuff and breccia, which contains shattered fragments of the surrounding rock milled by the eruption.
The science of pink diamonds
Argyle was the world's only consistent source of pink and red diamonds, accounting for over 90% of global supply. While diamonds from the mine came in various colors—including champagne, cognac, and blue—the pink stones are its most famous. Less than 0.1% of the diamonds recovered from the mine were pink.
The pink coloration is not caused by chemical impurities, which give color to other diamonds like nitrogen for yellow or boron for blue. Instead, the color is the result of a physical phenomenon known as plastic deformation. The intense pressures and temperatures experienced by the diamonds during their volcanic ascent to the surface are thought to have compressed their crystal lattice. This structural deformation alters how the diamond absorbs light, causing it to reflect a pink hue. This process creates visible pink bands or graining within the stones, a feature of Argyle diamonds.
The mine began alluvial operations in 1983, with the main open-pit mine being commissioned in December 1985. At its peak in 1994, it produced 42 million carats. Over its 37-year lifespan, the Argyle mine produced more than 865 million carats of rough diamonds before ceasing operations on November 3, 2020, due to the depletion of its economic ore reserves.