Which deposit involves gems that have eroded from the source rock and remained in place nearby?

The term that describes deposits involving gems that have eroded from the source rock and remained in place nearby is "eluvial." Eluvial deposits occur when minerals are transported by weathering processes, such as water movement and gravity, from the primary source rock but do not travel far from their origin. Instead, they accumulate in the vicinity of the source, often resting on or just beneath the surface.

Eluvial deposits are significant for gemology because they can contain valuable gems that have not been transported far from where they originated, making them easier to locate and mine. In contrast, alluvial deposits involve gems that have been transported over longer distances by water and typically are found in riverbeds or floodplains, while secondary and tertiary deposits refer more broadly to the process of minerals being deposited in various geological contexts over time, rather than specifically to their proximity to their source rock.