Kidney crystals are solid clusters of

Renal caliculi are the insoluble stones of salt crystals formed  in kidneys or ureters. The salt crystals in urine  like, calcium oxalate, uric acid, or calcium phosphate precipitate and solidify to form these stones. Conditions leading to formation of kidney stones include the ingestion of excessive calcium, low water intake, abnormally alkaline or acidic urine, and overactivity of the parathyroid glands.