This is usually a small oval stone, with rounded prominences regularly distributed over the surface. The colour varies from a light fawn to a deep brick red. It is heavy for its size, and of hard consistence. Uric acid is insoluble in water and dilute acids., but very soluble in caustic alkalies and weak solutions of alkaline carbonates. A convenient test is the murexid reaction. A fragment of the calculus is treated with a drop of strong nitric acid and heated. Effervescence occurs, and the heat is continued till a dry yellowish red residue remains. When caustic ammonia is added to the residue a bright violet red hue is developed.

Calculi formed of urates are rare, the salts being urates of ammonia and magnesia. They are small soft concretions formed in the kidney, and are distinguished by their solubility in boiling water. They hardly deserve to be reckoned among the vesicle calculi.