Diamonds are naturally occurring structures of pure carbon, formed under immense heat and pressure deep within the Earth. Finding one outside of established commercial mining operations is exceptionally rare, as diamonds are transported from the deep mantle to the surface by a unique geological process. This guide walks the amateur prospector through identifying the specific materials that may indicate a diamond’s presence.
Understanding Diamond Host Rocks
Diamonds form roughly 90 to 150 miles below the Earth’s surface, where intense heat and pressure allow carbon atoms to crystallize into their stable lattice structure. They remain trapped in the mantle until a rapid, violent volcanic eruption transports them to the surface. This eruption prevents the diamonds from reverting to graphite under lower pressure conditions. The molten material cools into two primary igneous host rocks: kimberlite and lamproite. These rocks form vertical, carrot-shaped structures, known as pipes or dikes, that extend deep into the Earth. Kimberlite is the more common diamond-bearing host rock.
Locating Essential Indicator Minerals
Host rocks are often deeply buried or weathered away, making direct location difficult. Prospectors instead focus on finding specific “indicator minerals” that are more abundant than diamonds but originate from the same deep-Earth environment. These minerals are released as the host rock erodes and are found scattered in stream beds or soil.
Key Indicator Minerals
One sought-after indicator is pyrope garnet, which displays a distinctive deep red to purplish-red color. Another key mineral is picro-ilmenite, a magnesium-rich form of ilmenite that appears as dense, opaque, glossy metallic-black grains. Unlike common magnetite, kimberlitic picro-ilmenite is non-magnetic. Prospectors also look for chrome diopside, easily recognizable due to its striking emerald-green color. Finding high concentrations of these three minerals suggests the material originated from a diamond-bearing source rock.
Amateur Field Prospecting Techniques
Once a promising area is identified, the next step is heavy mineral separation, which separates heavy minerals from lighter sediment. This process is typically performed on alluvial deposits in streams or riverbeds where the source rock has weathered. The material is first classified using stacked screens to separate the gravel into different size fractions. Water is then used, often with a gold pan or a specialized sluice box, to wash away lighter materials based on specific gravity. Diamonds have a specific gravity of about 3.52, which is higher than most stream sediment but lower than gold. Because of this relatively low density and the diamond’s non-wettable surface, they can be lost if the panning technique is too aggressive. The final concentrated material, known as “black sand,” is then carefully examined for indicator minerals and potential rough diamonds.
Confirming a Rough Diamond Find
If a clear or translucent stone is found, several non-destructive tests can suggest if it is a diamond or a common look-alike, such as quartz, zircon, or glass.
Non-Destructive Tests
Fog Test: Breathing on the stone causes temporary condensation. Because diamonds are exceptional thermal conductors, the fog on a genuine diamond will dissipate almost instantly, while a fake remains fogged for several seconds.
Water Drop Test: A diamond’s high specific gravity causes it to sink immediately and rapidly in a glass of water, whereas lighter imitations will sink more slowly.
Visually, a rough diamond often exhibits a greasy or oily luster and may have a characteristic octahedral or dodecahedral crystal shape. Transparent stones that allow text to be clearly read through them are highly unlikely to be diamonds, as a true diamond’s high refractive index usually distorts or obscures underlying printed words.