@jcrabapple Just some small addition: As a geologic term, the word sand specifies only grain size and not composition. The sand battery could very well be filled with sand, provided the crushed soapstone has the correct grain size.
In everyday language, "sand" is obviously understood differently, but I think this should at least be noted in a fact-check. Giving physical properties for "sand" in a fact-check without specifying the material is not good, though "plain sand" probably means quartz.