Alluvial deposits are defined as deposits where gems are eroded from their source rock and then transported away from the source and further concentrated in rivers and streams.

• A. True
• B. Not stated
• C. True
• D. Could be both

Answer: (C)

Alluvial deposits are placer-type deposits formed when gems are weathered out of their parent rock, carried away by moving water, and then sorted and concentrated downstream in rivers and streams. This is the classic process by which heavy minerals and gems accumulate in fluvial environments, making the statement an accurate description of alluvial (placer) deposits. The two True options simply reiterate the same idea, while alternatives like “Not stated” or “Could be both” don’t fit because the described mechanism is the defining characteristic of alluvial deposits in gemology.