Nicely dense foam.  This is NOT a universal product; didn't fit my playmakar massage gun.
My Playmakar massage gun ball heads came apart after just a few uses, so I decided I would take a chance and see if these fit despite the manufacturer saying probably not.  They do not; the diameter of the shaft is approx 1.5mm wider, so they don't fit in the playmakar.  And the rubber ring gasket is in a slightly different place.<br /><br />I didn't want to return them since I knew they were not supposed to fit, so I did a destructive teardown.  I liked what I saw; I found the plastic part inside the foam ball is much larger, and the foam stiffer, both of which should make these more sturdy than the playmakar model.
Rating: 5 stars