lapillus
noun
la·pil·lus lə-ˈpi-ləs 
plural lapilli lə-ˈpi-ˌlī
-(ˌ)lē
: a small stony or glassy fragment of lava ejected in a volcanic eruption
Example Sentences
Recent Examples on the Web This eruption would technically be a vulcanian eruption according to MVO and prevailing winds prevented ash or lapilli from falling on the eastern half of Montserrat. 
Erik Klemetti, WIRED, 8 Feb. 2010
Erik Klemetti, WIRED, 8 Feb. 2010 Word History
Etymology
Latin, diminutive of lapis
First Known Use
1747, in the meaning defined above