Lapilli is a form of tephra which is rocky material that falls to earth during a volcanic eruption. Lapilli literlly means little stones in Latin and are from 2mm to 64mm. Any bigger and they are volcanic bombs.
Lapilli is formed when droplets of molten lava are ejected during an eruption and cool as they pass throught he air. They take on a rounded shape caused by the movement through the air. Lapilli Tuffs are where large amounts of lapilli become welded together with residual heat. They are often formed from rhyolitic,andesitic and dacitic lavas. Accretionary Lapilii is where ash particles which bonded together within the eruption cloud- a bit like volcanic hail.