Image link is a bit broken, you need to add a .png to the end of imgur links or they won't work
fixed version:
As for what you can improve, try to work on your color gradients some. For particularly harsh lighting the way you shaded the (granite?) sphere there works, but for softer lighting the contrast shouldn't be as extreme.
As for your aliasing practice, in general you want the edges of your dark lines to be a darker color than the background, not a lighter one. Makes the edges look like compression artifacts.
Keep at it though, light and shadows can take some time to get used to, and I highly recommend doing a wide variety of shapes while doing it as it'll help you later on when you start drawing more complex objects. Cones, tubes, blocks, and more complex shapes like 20-sided die type shapes or curved "Paper" type objects are pretty handy for practicing these things in addition to the spheres you're drawing.
