Alluvial diamonds are secondary deposits that have been naturally separated from their original source and transported by water. Extensive erosion often results in higher quality diamonds because weaker stones broke apart during the journey. This guide offers practical steps for the prospector seeking to locate and identify these durable gemstones within a river environment. Success relies on understanding the geological processes of diamond transport and employing specific concentration techniques.
Understanding Alluvial Diamond Deposits
Diamonds are brought to the surface through volcanic eruptions that form kimberlite pipes, their primary source. Once exposed, the host rock weathers away, releasing the diamonds, which are then carried downstream by rivers. Because diamonds possess a high specific gravity, they are much denser than common river sediments like quartz and feldspar. This density causes them to settle out of the water column quickly when the current slows.
This sorting process concentrates diamonds in areas where the river’s energy decreases, creating natural collection points, or “trap sites.” Prospectors should focus on the deepest gravel layers resting directly on the bedrock, as the heaviest materials accumulate there over time. Excellent trap sites include the inside bends of a river, known as point bars, where the current is slack. Other effective traps are behind large obstructions such as boulders and bedrock riffles, or in natural depressions like potholes and crevices.
Recognizing Associated Indicator Minerals
Locating a diamond is challenging due to its rarity, but nature provides clues in the form of heavy mineral suites that travel with it. These “indicator minerals” originate from the same deep-earth rock as the diamond. They are generally easier to find because they are present in greater abundance, indicating that a diamond-bearing source is nearby and making the area a promising target for sampling.
The most important indicator minerals are pyrope garnet, ilmenite, and chromite, all dense and resistant to physical weathering. Pyrope garnet often appears as a reddish-purple, almost violet stone, sometimes featuring a kelyphite rim (a thin, greenish-black alteration layer). Ilmenite (specifically picroilmenite) is a black, metallic mineral significantly heavier than most other black sands. Chromite is another dense, black mineral found in the same geological environment, confirming the heavy mineral suite.
Essential Equipment and Sampling Techniques
Finding alluvial diamonds requires the proper equipment to process large volumes of gravel efficiently and concentrate the heavy material. Essential tools include a sturdy shovel, classifier screens, and a specialized concentrating device, such as a gold pan or a saruca. Classifier screens quickly sort the river gravel into manageable size fractions, preventing larger rocks from interfering with the concentration process. A common approach uses a quarter-inch mesh screen to remove oversize material and a sixteenth-inch mesh to concentrate the finer sands.
The physical sampling process begins by digging down to the deepest gravel layer directly above the bedrock, where the densest minerals accumulate. The gravel is placed into stacked screens and washed vigorously to remove silt and clay (wet sifting). The remaining classified material is transferred to a gold pan or a saruca, a conical pan designed for heavy mineral separation. Utilizing the high density of diamonds and indicator minerals, the pan is agitated underwater with a circular motion, allowing lighter material to spill over while the heaviest components settle at the bottom center.
Confirming the Identity of a Rough Diamond
Once the heavy mineral concentrate is obtained, the final step is to differentiate a rough diamond from common look-alikes like clear quartz or glass. Rough diamonds often exhibit a distinctive crystal structure, typically forming an octahedron (a double pyramid) or a dodecahedron (a twelve-sided shape). They will lack the brilliant facets of a cut stone. The surface is often unpolished, displaying a greasy or adamantine luster resulting from its high refractive index.
The specific gravity of a diamond (approximately 3.5 grams per cubic centimeter) makes it feel surprisingly heavy for its size compared to most other stones. As the hardest natural substance, a diamond resists scratching by nearly all other materials, making a scratch test a preliminary field identification method. Diamonds are also excellent thermal conductors. This property causes the stone to feel cool to the touch as it rapidly draws heat away from the skin.