grains of sand
Refers to individual particles of sand, often used in descriptions of texture or small quantities.
靴の中に砂粒が入って痛い。
A grain of sand got into my shoe and it hurts.
砂粒
Grains of sand were blown by the wind.
Compound of 砂 (すな, 'sand') and 粒 (つぶ, 'grain').