These K9 Crystal Clear Glass Balls are indeed Great for teleidoscopes. Very Clear and very nice and clear. No Known Flaws in the glass. approx 50 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 1/4 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.