Knucklebone by AroundSquare 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 brass and steel versions are both heavy, and carry a fair bit of momentum. The weight gives it a substantial feel, and the brass material is somewhat grippy, particularly if the hands are moist. The weight is a variable that affects play, and each material seems to be suited to different kinds of tricks.
The titanium version of the knucklebone is the jack of all trades. It is heavy enough to provide the momentum needed to reliably perform momentum-based tricks (think thumb rolls and sonics), while still being light enough to manipulate smoothly between two hands without exhausting the fingers too much. The titanium model has a sand-blasted finish that provides just the right amount of grip, while also helping to mitigate the stickiness that is sometimes felt when playing in humid climates or with damp hands.
The weight and hard material mean it will make a loud noise when it is dropped on hard surfaces, and has the potential to break things in its path. Be careful with it. The is a little softer than the steel, but both will still show scratches and dings on the drop portions (see photo with original prototype for reference).. it falls with weight, so it hits concrete hard.