Silica

Glass sponge skeleton is made of nearly pure glass (hydrated silica dioxide) and can be up to a meter in diameter. Glass sponge reefs found in the pacific ocean have unique soft tissue, or spicules, that fuse together into a robust scaffolding (Dictyonine) and are often referred to as Hexactinosan sponges.
Source: Glass Sponge ecology, Leys Lab