Picture: Crystal clusters and single crystals of calcium oxalate.   Magnification about 50X.

All beer contains calcium, which is introduced into the brewing process with raw materials such as grains, or in the brewing and blending water.  Oxalic acid, or oxalate salts, are present in hops and formed during the germination of barley, and in the course of malting.  The calcium ion and oxalic acid can form an insoluble compound called calcium oxalate (Ca₂CO₄). 

The solubility of the compound becomes less in cold conditions.  It can continue to form as the beer ages, or if it is stored at colder than recommended temperatures.   The resulting calcium oxalate precipitant can settle out of the beer and deposit on draught line surfaces. 

Calcium oxalate deposits, in combination with insoluble protein, and other material, is commonly termed "beerstone".  The beerstone will continue to build if the system is not cleaned properly or regularly; eventually it can lead to drawing problems and beer turbidity if it begins to flake off. 

Beerstone is evidenced as grayish/white or brownish deposits on the faucet or inner wall of the beer line, or as tobacco like flakes in the beer.

Note: The information in this section was provided by and reproduced with the permission of The Coors Brewing of Golden, Colorado.