Old English sand, from Proto-Germanic *sanda- (cf. Old Norse sandr, Old Frisian sond, Middle Dutch sant, German Sand), from PIE root *samatha- (cf. Greek psammos "sand;" Latin sabulum "coarse sand," source of Italian sabbia, French sable).
General Germanic, but not attested in Gothic, which used in this sense malma, related to Old High German melm "dust," the first element of the Swedish city name Malmö (the second element meaning "island"), and to Latin molere "to grind." Metaphoric for "innumerability" since Old English Sand dollar is from 1884, so called for its shape; sand dune attested from 1830.