A jewellery shop owner has two identical clear gemstones but cannot remember which one is diamond and which one is rutile. Rutile is a clear gemstone that appears identical to a diamond. It is often used as a cheaper alternative to diamond in jewellery. She is aware that the refractive index of diamond is 2.4 and the refractive index of rutile is 2.9. She has a machine capable of measuring the speed of light through each material and basic measuring equipment. Briefly describe how the shop owner can distinguish between the diamond and rutile gemstones. Use calculations to support your answer.