ABSTRACT
As we have seen in the previous chapter, the reflection of light plays an important part in the appearance of a gemstone. However, for identification purposes the most important single item of information about a gemstone is its refractive index (RI). This is because the RI of most gemstones is a constant which can be measured to two decimal places (and often estimated to the third decimal place). For this reason, many gems can be distinguished from each other with certainty even when there is very little difference in their RIs (e.g. natural and synthetic spinel; pink topaz and tourmaline).
