AO2's Knucklebone is the original hardbody knuckle roller, make no mistake. This is a simple skill toy that is manipulated with the fingers of one or both hands to perform tricks and patterns, or simply to pass the time. It is very discrete (unless dropped) and can be played in very restricted spaces, and in contexts where other showier skill toys may not be appropriate.
The kbone is very appealing, not only for its elegant form and ergonomic design, but also, for the range of play styles that it enables. The rounded lines are perfect for spinning, flipping, and passing it between the fingers. It can be manipulated very casually and quietly like a worry stone, or can be used to perform a surprising array of freestyle flows, technical tricks, isolations, and even aerials.
The delrin knucklebones, while relatively inexpensive, are prized among players for their versatility and feel. Unlike metal and other plastics, delrin has a warm feeling, almost reminiscent of wood. It is also very durable, and stubbornly resists dings and breakage. It will scratch, but because it is relatively light, it tends not to take (or inflict) heavy damage when it is dropped.
The weight and smoothness of the toy are variables that affect play, and each material seems to be suited to different kinds of tricks. The delrin version is conducive to quiet manipulation and two-play including transfers, twiddling between fingers, and two-handed rolls.
Warnings: These are solid--can break things or hurt things if dropped. It is recommended that you play over carpet, couch, or the like, or outside. Also, keep them away from young kids and don't let anyone try to ingest them.