From a bug report: fracture_rock() was unintentionally propagating
the boulder or statue's dknown flag to the resulting rocks, producing a
trivial but noticeable difference in description of "rock" vs "stone" if
the source object had been seen up close prior to being broken and the
rocks are then examined remotely or while blind.
The curse/bless state is propagating too, but this seems reasonable
so I've left it alone.
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