This is an EM style magic wall. It gets activated for a limited duration by
rocks or gems that fall on it. While activated, they can fall through it, and
rocks turn into emeralds, emeralds turn into diamonds, and diamonds turn into
rocks.
After the magic wall has stopped running, it cannot be activated again.

The duration in seconds for which magic walls are active is configurable.
A duration of zero will let the wall run forever.
All magic walls share the same timer.