Yellow sapphire’s attractive color is just one reason for this gemstone’s popularity. As with all sapphire, it’s also extremely durable, having a hardness of 9 on the Mohs scale. Sapphire has a specific gravity of 4.00, a refractive index of 1.76 – 1.78, and birefringence of 0.008.  The refractive index (RI), measured using a refractometer, is an indication of the amount light rays are bent by a mineral. Birefringence is the difference between the minimum and maximum RI. When birefringence is high, light rays reflect off different parts of the back of a stone causing an apparent doubling of the back facets when viewed through the front facet.