This is probably on <Someone>'s bug list, but I don't remember where
that lives. I found a copy of an old news message by him which pointed
out that gem probabilities are set for a given dungeon level at the time
it is being created, but they don't get reset when an existing level is
revisited. So giants' inventory creation and any monsters' death drops
generate gems using values from the level most recently made rather than
from their/hero's current location. That can lead to high level gems in
the main dungeon after entering the mines.
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