I thought about marbles on concrete as a subject and was happy to see you chose it as a subject. It's a nice marble and the strip of red in it is a great flash of color for this otherwise monotone picture. I'd like to see it a little more saturated. I'm not sure the dark and light helps the composition in this photo.
An interesting idea. I'm guessing that the placement of the shadow was intentional, but I'm not sure what the intent was. The transition from light to dark in the middle of the frame just seems distracting to me.
If that's supposed to be a crystal marble, I think you got ripped off. The light from it is dull, making it look like tarnished glass, plastic, or resin. The texture of the stone is nice, as is the shadow, and the red... something... adds an interesting note to the subject.