How do you get sand that is stuck on an agate off?

Answer 1

There are several possibilities, depending on the nature of the sand. If it doesn't come off with good old soap, water, and a toothbrush, you can either try a mild acid (following all safety instructions - protect eyes, skin, and lungs!) or you will have to remove it mechanically.

I'd start with vinegar, then pool acid, AKA muriatic, which is actually dilute hydrochloric acid. If it's not responsive to acid, try a diamond grinder of some sort, either a Dremel/Foredom tool, or a cabochon machine or flat lap. Don't inhale the dust.

A rock tumbler is another nice way to clean and polish agates.