Different gems or substances have varying refractive indexes. Diamond’s is unique in the sense that it has the high refractive index of 2.417. Cubic zirconia’s is about 2.15. The higher the RI (refractive index) the more ability it has to bend light. Light bends because the specific material structure slows it down as it enters. We see this in the dislocated image of an object immersed in a liquid, such as the apparent bending of a straw sitting in a glass of water. In vacuum conditions (such as interstellar space), light travels at approximately 186,000 miles per second, with an effective refractive index of 1. Once light hits our atmosphere, it slows down slightly due to a refractive index of 1.00029. Through diamond, light travels at closely 77,000 miles per second (186,000/2.417).