A Needle in a Haystack: How are Diamonds Detected in Rocks?
Kimberlite, a peculiar igneous rock best known for containing diamonds, also contains a variety of minerals with unique chemical compositions. About 6,400 kimberlite pipes (vertical columns of rock that come discharging when breaking the Earth’s surface) have been discovered worldwide, 30 of which were found to contain diamonds.
Indicator mineralogy is the best diagnostic tool for identifying diamond-bearing kimberlite. Certain indicator minerals typically occur within diamondiferous Kimberlites and are used as mineralogical tracers by prospectors, as they follow the indicator trail back to the volcanic pipe which may contain diamonds.
These minerals are rich in chromium or titanium elements which impart bright colors to the minerals. The most common indicator minerals are chromium garnets, eclogitic garnets, orange titanium-pyrope, red high-chromium spinels, dark chromite, bright green chromium-diopside, glassy green olivine, black picroilmenite and magnetite.
These minerals are generally absent from most other igneous rocks, making them particularly useful as indicators for Kimberlites.
In the past decade, technological advances have led to increased sophistication of determining indicator mineral chemistry for all indicator minerals.