How do stones form?

The details of stone formation are not fully understood yet and this is part of our ongoing research programme. It is hoped that if we can better understand this process we can develop new ways to prevent and treat stones. However, much has been described and this is summarised below.

Crystal formation in urine


crystal in jar

saturation zonesKidney stones are crystals made from chemicals found in the urine. In order for them to crystalize the urine must be saturated with these chemicals. A bit like growing a crystal in a jar in chemistry at school. The concentration of the chemicals that can form stones varies throughout the day.


"Zones" of varying risk are described. When the concentration is low crystals cannot form (undersaturated zone), at higher concentrations, crystals can form (metastable zone) and at higher concentrations crystals will certainly form spontaneously (oversaturated zone). We fluctuate between these zones throughout the day. Stone formers are more likely to spend longer in the higher concentrations zones. 


risk curves

risk of calcium stone formation


risk of recurrence



The important factors that influence stone formation are shown in this graph. The steeper the curves the more influencial the factors are in stone formation. The further from the midline the greater the difference from normal.

(graph reproduced courtesy of Dr Bill Robertson DSc)







Using Baysian maths a probabilty of stone formation (Psf) can be calculated. The Psf ranges from 0 to 1. The nearer to 1 the greater the stone risk. The graph on the right shows Psf scores for non-stone formers (left) and different types of stone-former (right). 

SSF - single stone former

RSF - recurrent stone former

dRTA - distal renal tubular acidosis

Enteric - enteric hyperoxaluria

Hereditary - inherited stone forming conditions

(graph reproduced courtesy of Dr Bill Robertson DSc)





Similarly, the Psf will predict the likelihood of recurrent stones. This graph shows that those with higher Psf scores have more stone episodes.


(graph reproduced courtesy of Dr Bill Robertson DSc)