- Fiberoptic Evanescent Wave Spectroscopy (FEWS) has been used for measurements of the absorption of very small (0.1mg) fragments of urinary calculi in the mid-IR. Such measurements were used for the determination of the chemical composition of each fragment. When large urinary stones are fragmented, it is possible to use this method for determining the chemical composition of the inner part and the outer part of the stone. We examined 40 urinary calculi and found that in 1/3 of them the inner part and the outer parts are identical. In 2/3 the inner part is different than the outer part. This change is not revealed by standard chemical methods that provide an average chemical composition. The novel FEWS method would be useful for the analysis of urinary calculi.