One of the intriguing properties of many gemstones is an optical phenomenon called pleochroism. It refers to gems which appear to have different colors or depth of color when viewed from different angles.
Pleochroism is caused by differing absorption of light rays in doubly refractive crystals. Thus no singly refractive gemstone can exhibit pleochroism. (Only diamond, spinel and garnet are singly refractive, however). Pleochroism is also absent in amorphous gems (those that have no crystal structure), opaque stones, colorless stones and most gems that are translucent rather than transparent.
Pleochroism occurs to varying degrees, called weak, distinct or strong. Among the gems that are strongly pleochroic are andalusite, iolite, kyanite, kunzite, sphene and tanzanite. Some pleochroic gems are said to be dichoric -- displaying two different colors; kunzite is an example. Others are said to be trichroic, such as andalusite.
Andalusite displays shades of yellow, olive and reddish brown depending on the orientation of the crystal. The pleochroic effect can be enhanced by specific orientation and cut. Those cuts with a long axis such as an oval, pear, marquis or emerald cut tend to show one color near the center and a second, usually darker color near the ends. Square and round cuts usually blend the colors into a mosaic.
Iolite was once known as water sapphire for its distinctive sapphire-like hue. But when viewed from different angles, iolite can appear clear as water, or a grayish yellow. When cut correctly, usually in an emerald or step cut, it will display a lovely violet blue with a velvety softness.
Kunzite displays two colors rather than three, but its pleochroism is quite pronounced and can easily be seen even in still photographs. The green form of spodumene, hiddenite, displays similar 2-color pleochroism.
While some gems, such as kyanite, displays different depths of color (light blue and dark blue), sphene is an example of a gemstone with dramatic pleochroism. Sphene displays colorless, green-yellow and reddish hues depending on the viewing angle. Since sphene has extraordinary dispersion or fire as well, it can put on quite a dazzling display.
Tanzanite is a strongly pleochroic gem but you will rarely see its pleochroism. That's because tanzanite is always heated to produce the familiar violet blue color. In its natural state tanzanite displays purple, blue and brown or yellow. But heat treatment removes the yellowish and brown tints and deepens the blue tone.