Nice selective desat... you can, unfortunatly see your reflection, but that is sometimes hard to mask. I like how you have filled the frame... Cropping in nice and tight.
Nice use of selective desaturation to answer the challenge. The tonality of the shot and black areas really give the subject nice contrast. One thing I found distracting was the reflection of the photographer in the sphere. Not sure if this was intentional? If not, perhaps layer masks in p-shop may have helped.
Reflection is distracting on the blue sphere. Might have tried wearing dark gloves, moving the camera down a bit more to be behind the stone, or even cloning out the reflection of your hand and camera. A neat collection of stones for this shot.