Aegirine is an important rock-forming mineral, widely occurring in various mafic, intermediate, and ultramafic rocks. Only a very small number of aegirine can be used as a gemstone. Due to the presence of polycrystalline twinning or the formation of fine interlayers of albite caused by solid solution separation, as well as the occasional presence of fine pores parallel to crystal planes, some transparent or semi-transparent varieties of aegirine can exhibit some special optical effects. Some have a milk-like luster and can be polished into moonstone; others have a halo effect, which is known as iridescent aegirine.