When blood becomes stagnant, its solidification by coagulation begins. In stagnant blood, by gravity, the heavier red blood cells settle preferentially to the deeper layer instead of the lighter platelets. This makes the deeper gelatinous layer of a clot that formed after death (when blood is stagnant) red – it resembles redcurrant jelly in appearance.
Reference:Underwood JCE and Cross SS (eds.), 2009, General and Systematic Pathology, 5th edition, Churchill Livingstone, Edinburgh, page 149. Go to reference
Red blood cell (erythrocyte), platelet (thrombocyte) and white blood cell (leukocyte)