I agree that it is a Pelt Rainbo--"Muddy"?
Backlight it, and backlight them all!
A "Muddy" will have a semi-opaque base glass.
The light will shine through somewhat and many will show some opalescence.
I know, hard to determine with the red but give it a try.
That is a very cool marble in my house at least.
Marble--On!!