oreguard

Replace blocks the player can’t see before the chunk data is sent out.

Only ores whose six faces are all enclosed by solid blocks are rewritten; blocks adjacent to air that the player could already see are sent as-is, so ores in caves remain real, and there are no "visible but unminable" ghost blocks.

The truth is automatically restored when the player approaches: periodically, plus "breaking one block immediately reveals the ores it was hiding," so mining feel matches vanilla. Unbreakable blocks such as bedrock are never disguised.

Three modes: hide ores (default), replace hidden ores with other ores, hide all solid blocks.

There is also an independent mining-behavior detection: it records "mining without any line of sight"; chain mining and area tools are recognized and exempted, and by default it only alerts administrators without taking any action.

Server-side only; no client installation required.