Alexandrite’s dramatic colour change is sometimes described as “emerald by day, ruby by night”. Other gems also change colour in response to a light-source change, but this gem’s transformation is so striking that the phenomenon itself is often called “the alexandrite effect”.
Alexandrite is also a strongly pleochroic gem, which means it can show different colours when viewed from different directions. Typically, its three pleochroic colours are green, orange and purple-red. However, the striking colour change doesn’t arise from the gem’s pleochroism, but rather from the mineral’s unusual light-absorbing properties.
Because of its scarcity, especially in larger sizes, alexandrite is a relatively expensive member of the chrysoberyl family. It shares its status as a June birthstone with cultured pearl and moonstone.