By Karen G Blaettler Updated Aug 30, 2022
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Diamonds captivate with their brilliance and are often seen as symbols of enduring love. While a polished gem showcases its full sparkle, an uncut rough diamond still holds the potential for that same radiance. Spotting these raw stones requires a blend of geological insight and hands‑on testing—methods honed by gemologists and miners alike.
Rough diamonds resemble weathered quartz pebbles but stand apart through location, crystal shape, specific gravity, hardness, grease adherence, and UV fluorescence. Combine these clues to identify an uncut diamond with confidence.
Most diamonds are tied to kimberlite pipes—ultrabasic igneous formations rich in olivine and devoid of quartz or feldspar. These pipes typically sit within ancient cratons, the planet’s most stable continental cores. While not every pipe hosts diamonds, the majority of discovered rough stones come from these settings. In unweathered, or “blue‑ground,” kimberlite, extraction demands crushing and separation. Weathered, or “yellow‑ground,” deposits can be processed with gold‑mining techniques like panning or sluicing.
Exceptions arise where deep crustal tectonics generate the high pressure and temperature needed for diamond formation—such as lamprophyre dikes in Canada’s Superior Province or Australia’s Argyle and Ellendale mines. Microdiamonds also appear in high‑pressure metamorphic rocks across China, Europe, Russia, and Indonesia, and rare meteorites. Regardless of origin, the common thread is a source of carbon and extreme conditions.
Diamonds crystallize in the isometric system, predominantly forming octahedral shapes. In contrast, quartz, the usual suspect in rough‑diamond confusion, develops hexagonal crystals that typically terminate at one end. While some quartz crystals (like Herkimer diamonds) show double ends, their hexagonal geometry remains a clear giveaway.
Diamonds boast a specific gravity of 3.1–3.5, significantly heavier than quartz’s 2.6–2.7. In placer deposits, tumbled quartz pebbles and diamonds may appear alike, but density separation tools—pans, sluices, or shaker tables—can distinguish them. On a properly set shaker table, quartz settles toward the center while the denser diamonds rise to the edge.
Ranking 10 on the Mohs Hardness Scale, diamonds can scratch every other naturally occurring mineral, but only diamonds can scratch other diamonds. Quartz, a common confounder, scores 7. Commercial hardness kits often stop at corundum (9); any material that corundum cannot scratch is a diamond. Tests should be conducted on fresh, unweathered surfaces to avoid false negatives, as weathering can slightly reduce apparent hardness.
Minerals generally resist grease, allowing a simple yet effective separation: lay a slurry over a greased table, and the diamond will cling while other particles slide away. Moreover, roughly 30% of diamonds fluoresce under shortwave ultraviolet light, usually emitting a light blue glow but sometimes appearing white, yellow, orange, or red. Avoid destructive tests like checking for cleavage; intentionally breaking a sample risks damage.
