Hello! I am right now doing a logo comission and im trying to achieve this kind of effect(The marble): 
I tried drawing it but it didnt help, any feedback?
I’m not completely sure about how well this will work, but you can try adding more shadows/shines to make a rounded effect. Instead of using a gradient on the ball, try to make it shaded on a curve to give a rounded effect.
the marble in the flamingo thumbnail is more transparent towards the middle and has shine and the green has you know that it is a marble since marbles usually have multiple colors
I believe you should add shine towards the curves of the marble along with the underside of it, just as it shows in the reference. (to give the shine a more crisp look, erase the tips and width of it) In addition, airbrush along the curves as well to give a somewhat depth. The middle has more of a transparency which can be used with the colour’s of green, purple and blue. (you can simply reduce the transparency on that layer and erase the middle with a leveled transparent eraser)