Most people probably thought Acrylic when they saw the picture of the Cyrstal Ball.
That often happens as clear is clear it is the feel of crystal that is different especially the weight and surface tension.
Crystal is glass.

Clear and Transparent Acrylic Warning

This is a warning not to leave any transparent acrylics especially clear acrylics in direct sun.

Always cover acrylics when you are in the sun so that you do not damage the ball or your property.