Also depends what kind of stability you're talking about. If you mean, e.g., a hard ball lying on the flat surface -- it's relatively hard to crush (harder than a cube of the same material), but very very easy to roll (unlike a cube of the same material).
was thinking about the yoga ball, that i used as chair, that i am sitting on top at the moment. i pushed it around, pounced on it, stretched my back, pushing it into walls, and so on. it always bounced back and/or diverted by power elsewhere. it's firm yet yielding. it flexed and held its shape. the only thing i have not tried yet is wearing leotard while using the ball.
a thought occur to me (i know random thought just pop in and out, must have been because of too much empty space) that if you spin the yin-yang symbol on its axis, it would be a yin-yang sphere, which doesn't really mean anything at all.