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Certain minerals are present in the rocks from the upper mantle that occur
with diamonds in kimberlite and lamproite pipes, as seen in nearby cases
of xenoliths and diamond inclusions. Some of these minerals, being resistant
to weathering and denser than quartz sand, concentrate in channel bottoms. Because they occur in far greater abundance than diamond, exploration
geologists look for these "indicators" among the gravel of regions they suspect may host diamond-bearing pipes.
Indicator minerals for diamond include, in order of decreasing significance: garnet, chromite, ilmenite, clinopyroxene, olivine, and zircon. But the order of persistence in streams is zircon, ilmenite, chromite, garnet, chromian diopside, and olivine. Diamond itself is obviously a most important indicator.
Most indicator minerals have a distinctive color. Seen here are red pyrope garnets, green chromian clinopyroxene, black ilmenite and chromite, and yellowish-green olivine. Photo by George E. Harlow.
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