Casio has developed lenses made out of transparent ceramics for use in their digital cameras. Apparently using this material can reduce the overall size of their telephoto lenses by nearly 20% because of their higher refractive index. Most crown glasses have refractive indices of around 1.5, while this material has a refractive index of 2.08. Transparent ceramics have many of the same properties of sapphire, but at a greatly reduced cost. You can look here for more details, without the corporate self-promotion. The military applications are rather obvious, as are their uses for fireplace glass.