They appear grey-red with alternating light and dark lines (known as lines of Zahn) which represent bands of white blood cells and red blood cells (darker) entrapped in layers of fibrin. Cause
Virchows triad describes the pathogenesis of thrombus formation:
Endothelial injury: Injury to the endothelium (interior surface of blood vessel), causing platelet activation and aggregation;
Common causes include: trauma, smoking, hypertension, atheroma. Hemodynamic changes (stasis, turbulence):  Blood stasis promotes greater contact between platelets/coagulative factors with vascular endothelium. If rapid blood circulation (e.g., because of tachycardia) occurs within vessels that have endothelial injuries, that creates disordered flow (turbulence) that can lead to the formation of thrombosis;Common causes of stasis include anything that leads to prolonged immobility and reduced blood flow such as: trauma/broken bones and extended air travel. Hypercoagulability (also called thrombophilia; any disorder of the blood that predisposes to thrombosis);
Common causes include: cancer (leukaemia), factor V mutation (Leiden) - prevents Factor V inactivation leading to increased coagulability.Disseminated intravascular coagulation (DIC) involves widespread microthrombi formation throughout the majority of the blood vessels. This is due to excessive consumption of coagulation factors and subsequent activation of fibrinolysis using all of the bodys available platelets and clotting factors. The result is hemorrhaging and ischemic necrosis of tissue/organs. Causes are septicaemia, acute leukaemia, shock, snake bites, fat emboli from broken bones, or other severe traumas. DIC may also be seen in pregnant females.
Classification
Thrombi are classified into two major groups depending on their location and the relative amount of platelets and red blood cells (RBCs). The two major groups are:

Arterial or white thrombi (characterized by predominance of platelets)
Venous or red thrombi (characterized by predominance of red blood cells). Microclots
In the microcirculation consisting of the very small and smallest blood vessels the capillaries, tiny thrombi known as microclots can obstruct the flow of blood in the capillaries. This can cause a number of problems particularly affecting the alveoli in the lungs of the respiratory system resulting from reduced oxygen supply. Microclots have been found to be a characteristic feature in severe cases of COVID-19, and in long COVID. Mural thrombi
Mural thrombi are thrombi that adhere to the wall of a large blood vessel or heart chamber. They are most commonly found in the aorta, the largest artery in the body, more often in the descending aorta, and less often in the aortic arch or abdominal aorta. They can restrict blood flow but usually do not block it entirely. Mural thrombi are not usually found in vessels already damaged by atherosclerosis.A mural thrombus can affect any heart chamber. When found in the left ventricle it is often a result of a heart attack complication. The thrombus in this case can separate from the chamber, be carried through arteries and block a blood vessel.