Synthetic forsterite has also been sold as tanzanite, and presents a similar appearance. It can be distinguished from tanzanite in three ways. The first is by using a refractometer: forsterite will show a refractive index of between 1.63 and 1.67, while tanzanite will show a higher index of 1.685 to 1.707. The second way is by using a Hanneman filter: through it, genuine tanzanite will appear orange-pink, while forsterite will appear green The third way is by examining a cut stone through its crown facets and viewing the pavillion cuts at the back of the stone using a standard jeweler's loupe: forsterite will show birefringence, making the pavillion cuts appear "doubled up", while the much lower birefringence of tanzanite will not have this characteristic..