A blood specimen processor by means of which blood specimens are collected into a collection tube containing an anticoagulant and a molded buoy of predetermined density between 1.045 and 1.084 with an embedded water swellable o-ring that expands to form a robust seal in a leucocyte density gradient between the buoy's outer surface and tube's inner surface. The buoy is made of a first resin and a second resin, the first resin having a lower density than the second resin, the first resin and the second resin being present in such amounts and relative proportions so that the body has an overall density between 1.045 and 1.084 and preferably, a center of gravity below a geometric center of the body. When the blood specimen contained in the processor is centrifuged, the buoy is thrust through the separating blood and the developing interface of erythrocytes, leucocytes, and plasma. A reproducible gradient develops that builds a reproducible buffy coat ring around the upper trough at the junction of the edge of the separator buoy and the tube wall. The relative proportions of resin are controlled to create a separator buoy of a desired density so that after centrifugation the buffy coat or other cells of interest will be above or below the water swellable o-ring.