What does 'placer' refer to regarding gem deposits?

'Placer' refers specifically to workable alluvial deposits of gem material. This term is used to describe concentrations of precious stones and minerals that have been eroded from their original source and transported by water, often accumulating in riverbeds, beaches, or floodplains. These deposits are typically easier to mine and extract than those found in hard rock formations, making them significant in the gem and mineral extraction industry.

In contrast to other choices, fractured rock formations denote structural geology rather than deposits of gems. Artificially created gem deposits imply synthetic origins, which differ from natural placer deposits. Lastly, mineral concentrations found in igneous rock relate to minerals that have crystallized from molten magma, which does not align with the concept of placer deposits that form through weathering and sedimentation processes. Thus, the definition of 'placer' specifically relates to those alluvial deposits where gem materials can be efficiently mined.