Sometimes small stones are discharged as gravel, forming smooth, round, greyish balls like hemp seeds. The calculus proper is mostly of an irregularly spherical shape, tuberculated on the surface like a mulberry, and of a greyish or nearly black colour. On section it is seen to be in layers, some of the latter generally composed of uric acid, which usually forms the nucleus. The calculus contains abundant organic material which holds the colouring matter, so that when the oxalate of lime is dissolved out the organic basis often retains the shape of the calculus. Oxalate of lime is insoluble in the alkaline carbonates and organic acids, but soluble in nitric and hydrochloric acids. If a fragment be heated on a piece of platinum before the blow-pipe it becomes black, swells up and leaves a bulky white ash of caustic lime which gives a strong alkaline reaction with litmus.