Twinning is rarely observed. Biotite shows substantial variability in chemical composition, and thus, variable optical properties. The interference color is often masked by the mineral's color. Biotite often has Zr halos as radioactive disintegration products. As micas weather, they become more difficult to identify.
Birefringence is generally high and moderate and high relief is typical.
Many pyroxenes exhibit parting, commonly parallel to the 100 or 001 planes. Twinning may also be present. Enstitite is usually colorless, but hypersthene is frequently pleochroic in shades of pink to blue-green. Color and pleochroism increase with iron content. In the absence of diagnostic criteria, it is preferable to use the general term orthopyroxene or clinopyroxene rather than the more specific mineral name.