Which statement best characterizes secondary deposits compared with primary deposits?

• A. Diamonds are more concentrated in secondary deposits and secondary deposits have a higher percentage of gem-quality diamonds.
• B. Diamonds are more concentrated in primary deposits and secondary deposits have a higher percentage of gem-quality diamonds.
• C. Diamonds are more concentrated in secondary deposits and primary deposits have a higher percentage of gem-quality diamonds.
• D. Diamonds are equally concentrated and gem-quality percentages are equal.

Answer: (A)

Diamonds reach secondary deposits by being eroded from their original host rocks and carried by water until they settle in placer (alluvial) deposits. The moving water acts as a strong concentrating force, so heavy diamonds accumulate in limited areas such as riverbeds and gravels, creating a much higher local concentration than in the primary source rocks. During transport, hydraulic sorting and abrasion tend to remove damaged or smaller stones, leaving a larger fraction of intact, robust gems. This combination—greatly concentrated deposits and a higher proportion of gem-quality stones—makes secondary deposits stand out compared with primary deposits, where diamonds are dispersed within host rock and yield a broader mix of quality.