I have never used sand for tumbling rocks but have for tumbling old
pennies etc. It works well on the pennies..not too abrasive bu does
the job. Remember, sand is mostly quartz and therefore is about a 7
on Mohs scale. Therefore, it would only work on stones softer than 7.
I have seen stones and shells on some beaches tumbled by the sand
that were as shiny as any produced by a tumbler. Unfortunately, we
must talk in geologic time as to the time it takes to get the job
done. Probably 2 to 4 times that necessary to do it with Si2 grit.
Good luck. Cheers from Don at The Charles Belle Studio in SOFL
where simple elegance IS fine jewelry! @coralnut2