These Crystal Clear Glass Balls are indeed Great for teleidoscopes.
Very Clear and very nice and clear.
No Known Flaws in the glass.
approx 25 MM in diameter
This size is great for an approx 1 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.