need you have asked?
2 things remove mildew stains/kill spores: UV light and chlorine bleach.
Massive amounts of hydrogen peroxide and/or oxygen bleach might also have an effect.
Whether or not this may inadvertantly effect your UNKNOWN TYPE of “river rock” can not be determined. Any old stone recovered from a river (universally considered rounded or “polished” by the tumbling action of water force and/or errosion can be called river rock!).
Silly ambiguous questions deserve condensending replies!
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