Siliceous sinter

A silica-rich rock deposited from rapidly cooling watery solutions. Often irregularily laminated, but also sponge-like or massive. May form contiguous crusts or isolated, often rounded bodies. Usually made of opal (eg. opal-AG, opal-CT), but may also contain or be exclusively made of chalcedony. Silicious sinter typically forms at hot springs, and often called geyserite, but silicious rocks and minerals of similar porous to laminated texture and composition can form under comparable conditions in subsurface veins and are sometimes also called silicious sinter.