Primary gem deposits occur in places where gems formed or came to the surface.

• A. In deep ocean trenches
• B. In desert dunes
• C. In glacial frost pockets
• D. In places where gems formed or came to the surface

Answer: (D)

Primary gem deposits are the places where the gemstone actually forms in the host rock or where the gem’s source rock is located. This means the mining happens near the origin of the gem, not where it’s later concentrated. The statement that gems formed or came to the surface captures that origin directly, making it the best description of a primary deposit. The other settings describe secondary or placer deposits, where gem fragments have been eroded from their source and then concentrated in sediment like desert sands or glacial deposits; deep ocean trenches aren’t typical gem-forming environments for the common gems.