per·lite also pearl·ite 
(pûrlīt′)
(pûrlīt′)n.
A natural volcanic glass similar to obsidian but having distinctive concentric cracks and a relatively high water content. In a fluffy heat-expanded form perlite is used as a lightweight aggregate, in fire-resistant insulation, and in soil for potted plants.
[French (from perle, pearl, from Old French; see PEARL1) or German Perlit (from Perle, pearl, ultimately from Vulgar Latin *pernula).]
per·litic (pər-lĭtĭk) adj.