Alluvial Deposit is defined as a deposit where gems are eroded from their source rock and transported away from the source and further concentrated in rivers and streams.

• A. A deposit where gems are eroded from their source rock and transported away from the source and further concentrated in rivers and streams.
• B. A deposit formed by lava flows on the surface
• C. A deposit of sedimentary rock only found in deserts
• D. A deep mantle-derived deposit

Answer: (D)

Alluvial deposits form when material is broken down from its source rock and carried by running water, then dropped and concentrated where the flow slows. For gems, this means minerals are eroded and transported by rivers or streams, eventually settling in places like riverbeds, bars, or floodplains where heavier grains accumulate. This hydraulic sorting by water is the hallmark of alluvial (placer) deposits. The other descriptions point to processes not involving water-lain sedimentation: lava-flow surfaces come from molten rock, desert sedimentary deposits form in arid, often wind-driven conditions, and mantle-derived sources originate deep underground. So the idea of gems being eroded, transported away, and concentrated in rivers and streams fits an alluvial deposit.