CAUTION: THIS IS ONLY THE SPHERE CORE!! This a spherical core for Mefferts' Mosaic Cube, designed by me. Several people have noticed that the mechanism is a bit "spongy". The issues are illustrated by Roxanne Wong in this YouTube video.
The cause of this spongyness, and the resulting less-than-optimal turning, is the large cavity inside the puzzle into which the pieces are pushed. Twisty Puzzle Forum contributor Burgo suggested to fill the cavity with a spherical core. This new spherical core fills the cavity completely, making the puzzle more stable.