Other forms: sintered; sintering
Sinter is a rocky material that forms around geysers and hot springs. The surface of sinter is textured and porous-looking, but it feels as hard as rock.
Sinter forms when extremely hot water from geysers and hot springs meets cooled waters of their surrounding discharge aprons, resulting in a crust made of various minerals, mainly silica. To create this kind of deposit is to sinter, a process that also happens artificially, when plastic, metal, or ceramics are being manufactured. The word comes from German, and it shares a root with the English cinder.