The black/white coloring on the practice stones makes them easier to see during flight and to find in the field afterwards.
The sling pictured here is a five strand jute sling with tapered cords.
EDIT: November 8, 2012
I just picked up my newest batch of clay slingstones from the pottery shop today.
The clay used for this batch was more dense than my first batch which gave a slightly heavier stone at a slightly smaller size. The stone on the right is from the first batch.