Lapilli is a size classification term for tephra, which is material that falls out of the air during a volcanic eruption. Lapilli (singular: lapillus) means "little stones" in Latin. They are in some senses similar to ooids or pisoids in calcareous sediments.
By definition lapilli range in size from 2 mm to 64 mm in diameter. A pyroclastic particle greater than 64 mm in diameter is correctly known as a volcanic bomb when molten, or a volcanic block when solid. Pyroclastic material with particles less than 2 mm in diameter is referred to as volcanic ash.
Lapilli tuffs are a very common form of volcanic rock typical of rhyolite, andesite and dacite pyroclastic eruptions. Here, large thicknesses of lapilli can be deposited during a basal surge eruption. Most lapilli tuffs which remain in ancient terrains are formed by the accumulation and welding of semi-molten lapilli into what is known as a welded tuff.
The heat of the newly deposited volcanic pile tends to cause the semi-molten material to flatten out as they become welded. Welded tuff textures are distinctive (termed eutaxitic), with flattened lapilli, fiamme, blocks and bombs forming oblate to discus-shaped forms within layers. These rocks are quite indurated and tough, as opposed to non-welded lapilli tuffs which are unconsolidated and easily eroded.
Rounded tephra balls are called accretionary lapilli if they consist of volcanic ash particles. Accretionary lapilli are formed in an eruption column or cloud by moisture or electrostatic forces, 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.
Armoured (or cored) Lapilli
This lapilli is a variety of accretionary lapilli, though contains lithic or crystal cores which are coated by rinds of coarse to fine ash. Armoured lapilli ONLY occurs in hydroclastic eruptions where significant moisture is present. The vapour column contains cohesive ash which sticks to particles within it.