Lapilli

: lapillus) is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts.

These rocks are quite indurated and tough, as opposed to non-welded lapilli tuffs, which are unconsolidated and easily eroded.

Rounded balls of tephra are called accretionary lapilli if they consist of layered volcanic ash particles.

Accretionary lapilli are formed by a process of wet ash aggregation due to moisture in volcanic clouds that sticks the particles together, with the volcanic ash nucleating on some object and then accreting to it in layers before the accretionary lapillus falls from the cloud.

Accretionary lapilli are like volcanic hailstones that form by the addition of concentric layers of moist ash around a central nucleus.