Wikipedia
Urostealith is a fatty or resinous substance identified by the Austrian chemist J. F. Heller in 1845 as the main constituent of some bladder stones.
Giuliano dall'Olio (2008), Nuovo componente dei calcoli vescicali — L’ “urostealite ” di Heller. (In Italian) RIMeL - IJLaM, volume 4, issue 1, Società Italiana Medicina di Laboratorio. Online version accessed on 2009-07-30.
According to Heller's and other contemporary descriptions, urostealith is a soft brown substance, insoluble in water, sparingly soluble in alcohol and easily soluble in ether. Upon heating it softens at first, then expands and carbonizes before melting. It dissolves in solutions of sodium carbonate, and the latter was successfully used by Heller to dissolve and break up stones in a patient's bladder.
Urostealith stones seem to be very rare.
S. Materazzi, R. Curini, G. D'Ascenzo, and A. D. Magri (1995), TG-FTIR coupled analysis applied to the studies in urolithiasis: characterization of human renal calculi. Termochimica Acta, volume 264, 75--93.
The circumstances that lead to their formation, as well as the composition of the substance, are still obscure, and little has been published on the topic.
Edward Wagman; Anthony Barbara; James Marquis; Marvin Chirls; Anita Falla (1966), Renal Fat Embolization and Urostealith Formation Complicating Femoral Fracture. J. of the American Medical Association, Vol. 198, Issue 7, 721-723 Online version accessed on 2009-07-30.