what to do with fraction as index number?

I have a question about sapphires and their refractive index. I have the refractive index of sapphires to be 1.762 - 1.778 other sites vary slightly. Do all sapphires have the exact index number, or do they vary slightly from stone to stone?

• Answer:

  Your question answers itself. You state that the refractive index is 1.762 - 1.778. That is a range rather than a single number. The reason it is a range is because refractive indices do vary from one stone to another. The refractive index of a mineral depends on the elements from which it is made. Sapphires are a variety of the mineral corundum. In its ideal from (Al2O3) corundum would be colorless. However, sapphire have other elements substituting for aluminum within its structure. (This is what creates the colors.) As varying amounts of these other elements would cause differences in the refractive index.