Gemstones have a higher density than air, therefore when light enters the gemstone it will slow down and change direction, resulting in refraction. The ratio between the speed of light in the air and inside of an object is called the refractive index . Refractive indices in gemstones can be categorized into single and double refractions. The crystals of isometric gemstones, being isotropic in their atomic arrangements, exhibit single refraction since the velocity and direction of lights inside these gemstones remain the same; all other types of crystal systems are anisotropic, meaning that the velocity of light differs in the direction in these gemstones. Tetragonal and hexagonal gemstones have 2 refractive indices; orthorhombic, monoclinic and triclinic gemstones have 3 refractive indices. Whether it is 2 or 3 refractive indices are collectively referred to as double refraction , and the differences between each index are referred to as birefringence . Most gemstones have consistent refractive indices, except for emeralds, which may vary depend on the locales of the deposit. The degree and category of the refractive index are therefore important parameters in gemstone appraisal. Refractometer, a simple optical instrument, is used to measure the refractive index of a gemstone.