What are renal calculi and what conditions may favor their formation?

Kidney stone or nephrolithiasis or urolithiasis is a condition when a stone develops in the urinary tract (kidneys, ureter or urethra). Typically they are formed in kidneys and may leave them in the urine stream. The males are more prone to have them because high estrogen, inhibitory effect by estrogen on kidney stones,  high muscle mass favors kidney stone formation. 

Nephrolithiasis is formed primarily from calcium and oxalate salts.  Calcium oxalate forms the The supersaturation of urine with these salts are responsible for the formation. The mechanisms involved in the formation are-

  Nucleation of the crystal structure- There will be a nidus of crystal formed due to supersaturation of urine with the salts like calcium and oxalate.

Crystal growth- The crystals will grow around the nidus causing the crystals of these salts to increase in dimension.

Aggregation and secondary nucleation- This will further increase the dimensions forming kidney stone.