Kaleidoscope Making Parts, K9 Crystal Clear Glass Balls, Spherical Collimating Lenses 48.8mm.

These K9 Crystal Clear Glass Balls are indeed Great for teleidoscopes.
Very Clear and very nice and clear.
ssome flaws in glass such as 2 known air bubbles
approx 48.8 MM in diameter. Crystal Ball for Teleidoscopes, Photography, Props Decoration, K9 Crystal Lens Ball Suitable for Gazing
This size is great for an approx 2 1/2 inch ID diameter tube.

Made for the laser and Fiber optic industry these are called Spherical Collimating Lenses
The prices each reflect they are indeed lenses and not just Crystal Balls or not just Clear marbles.