Good thinking!
Filter sand should be quartz (unless you've got some sort of filter goop in your filter). With a digital macro photo OR under a hand lens, you should be able to see if the sand particles are quartz bits and crystals. Also, if you put bleach in the jar -- if it 'dissolves' or lightens the particles overnight, it's not real sand.
What sort of pool surface do you have? Painted concrete? If so, it's just epoxy chalking (that's how it wears!).What about the cloudiness that we sometimes see after brushing?
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