Most diamonds found at or near Earth’s surface were delivered from the mantle during deep-source volcanic eruptions. These diamonds occur in vertical igneous structures known as pipes, which are often composed of kimberlite or peridotite.

These pipes are difficult to locate. Their surface exposure is usually covered with soil and vegetation, and it might be only a few acres in size. The pipes are often found by searching soils and sediments for mineral grains that are characteristic of the pipe but absent in local surface materials. Small particles of chromium-rich diopside are bright green in color, are often abundant in the pipes, and are easy to recognize in surface materials.

Geologists use these green diopside fragments to locate the pipes. They know that the fragments are liberated as the pipe weathers, then scattered by the actions of mass wasting, streams, and glaciers. When diopside fragments are discovered, the geologist knows that they originated up-slope, up-stream, or up-ice from the location in which they were found.

A trail of diopside fragments can lead the geologist to the pipe from which they were weathered. This activity, known as “trail-to-lode” prospecting, finds many diamond pipes and an even larger number of pipes without diamonds.

Note: It would be almost impossible to locate pipes by looking for diamonds. Diamonds make up such a small fraction of the overall rock in the pipe, and weathering debris from the pipe is then mixed into local rock debris. An exceptional pipe might contain a couple carats of diamond per ton!