The Milky-way rising over the Tufas - Mono Lake, CA
Tufa is a variety of limestone formed when carbonate minerals precipitate out of ambient temperature water. Geothermally heated hot springs sometimes produce similar (but less porous) carbonate deposits known as travertine. Tufa is sometimes referred to as (meteogene) travertine, it should not be confused with hot spring (thermogene) travertine. Tufa, which is calcareous, should not be confused with tuff, a porous volcanic rock with a similar etymology that is sometimes also called "tufa".