It is a precipitate from limestone comprising aggregates of fine crystals of varying composition, usually made of carbonates such as calcite, aragonite, hydromagnesite, and/or monohydrocalcite.

As microbial colonies grow, they trap and accumulate chemically precipitated crystals in an organic matter-rich matrix.

These heterotrophic microbes, which produce CO2 as a waste product of respiration and possibly organic acids, may help to dissolve the carbonate.

[citation needed] In 2017, archaeologists at the University of Chinese Academy of Sciences in China discovered a bronze jar dating back over 2,700 years, containing animal fat combined with moonmilk.

[3] The world's largest formation of brushite moonmilk is found in the Big Room of Kartchner Caverns State Park in southern Arizona.