40mm - Fiber Optic Cat's Eye Marble (Approximately 1-3/8") Cobalt/Dark Blue. We usually carry only glass marbles in our stores, but these Fiber Optic marbles are really beautiful and we have decided to add them to our collection of marbles that we offer. They are brilliant spheres made from fiber optic telecommunications material. When placed underneath any light, they shine like a cats eye gemstone. We have often heard people say that pictures of these marbles do not do them justice. We have found that they are right. These marbles are so much more brilliant when seen and held in person.