... -Al oxides, as containing magnesium, iron, zinc, manganese, etc. elements, they can be divided into many forms, such as aluminum ... spinel, chrome spinel and so on. Due to the different elements, different spinel can have different colors, for example, magnesium ... index: 1.718, it changes according to the content of trace elements; maximum is 2.000; no birefringence; no polychroismSpecial optical ...