Although most of the garnets found at Earth’s surface have formed within the crust, some garnets are brought up from the mantle during deep-source volcanic eruptions. These eruptions entrain pieces of mantle rock known as “xenoliths” and deliver them to the surface in a structure known as a “pipe.” These xenoliths are the source of most diamonds found at or near Earth’s surface.

Diamond pipe: Simplified cross-section of a diamond pipe and residual soil deposit showing the relationships of xenoliths and diamonds with the pipe and residual soil.

Although xenoliths contain diamonds, they often contain a tremendous number of garnets for every diamond, and those garnets are generally larger in size. These deep-source garnets are very different from the garnets that form in the crust at shallow depth. So, a good way to prospect for diamonds is to look for these unique garnets. The garnets serve as “indicator minerals” for geologists exploring for diamond deposits. As the xenoliths weather, their garnets are liberated in large numbers. These unusual garnets then move downslope in soils and streams. Geologists who find them can follow the garnet trail to the source deposit. Some of the diamond pipes in Canada were found by following a garnet trail produced by moving ice.