lapillus

The word lapillus comes from Latin and means "little stone" or "small pebble." In its original sense, it could refer to a small stone in general. In modern English, though, the word is used mainly in geology and volcanology.

Think of a volcano erupting and throwing material into the air. The finest particles are called ash. The much larger pieces are called blocks or bombs. Between those two sizes are lapilli, the plural form of lapillus. In standard geological use, they measure about 2 mm to 64 mm across. These fragments may be made of lava that cooled in flight, or of older rock broken from the volcano during the eruption.

When these pebble-sized pieces fall back to the ground, they can build up in layers around the volcano and become part of a volcanic deposit. Because of that, the term often appears in descriptions of eruption debris, crater deposits, and rock layers formed by explosive volcanic activity.

So the story of lapillus is fairly simple: it began as the Latin word for a little stone, and in modern use it usually points to a small volcanic fragment, larger than ash and smaller than the biggest chunks thrown out by an eruption.