Sapphire and Ruby ball lenses are both made from Al203. Ruby or Ruby-Doped sapphire owes its red color to traces of chromium oxide (chromium content for ruby balls is typically >0.5%). While their physical and chemical properties are basically the same, their optical properties are somewhat different. Sapphire has superior optical transmission qualities. Ruby Balls are easier to see and therefore easier to handle for physical applications.
For general information about ball lens, as well as how to calculate the NA and Focal Length, view Understanding Ball Lenses.