Primary deposits contain gems still embedded in the host rock where they crystallized—kimberlite pipes for diamonds or metamorphic schists for corundum. Mining these requires crushing or tunneling hard rock.
Secondary (alluvial or placer) deposits form when weathering frees gems from primary sources; rivers, waves, or gravity then concentrate the denser, more durable stones in gravels or beaches. Sapphires and rubies are commonly recovered from both types. Secondary deposits are often easier and less expensive to mine but may show rounded crystal edges from transport abrasion. Gemologists examine surface wear and inclusion patterns to infer deposit type.