From what substance is blood clot actually formed?

Blood clots are formed from a protein called fibrin, which is produced when a protein in the blood called fibrinogen is converted to fibrin by the action of an enzyme called thrombin. Thrombin is produced when another enzyme called prothrombin is activated by a complex series of reactions known as the coagulation cascade. The coagulation cascade is triggered when the blood comes into contact with a damaged blood vessel or foreign surface.