Rough diamonds may sometimes be mistaken for worthless pebbles

In their natural uncut and unpolished state many diamonds could indeed be overlooked in the mineral material surrounding them. Years of study and observation have lead geologists to believe that the diamonds in all of Earth's commercial diamond deposits were formed in the mantle and delivered to the surface of our planet by deep-source volcanic eruptions. These eruptions produce the kimberlite and lamproite pipes that diamond prospectors search for. The diamonds weathered and eroded from these eruptive deposits are now found in the sedimentary (placer) deposits of streams and coastlines.