Final answer:
Labradorescence is an optical phenomenon that causes a broad flash of color in labradorite feldspar. It disappears when the gem is moved. It is caused by the presence of lamellar intergrowths within the feldspar crystals.
Step-by-step explanation:
The broad flash of color in labradorite feldspar that disappears when the gem is moved is known as labradorescence. Labradorescence is an optical phenomenon where light enters the gem and is reflected back out in a colorful iridescent display. It is caused by the presence of lamellar intergrowths within the feldspar crystals.
Labradorescence is commonly seen in labradorite, a type of feldspar. When labradorite is viewed from different angles, the colors can shift and change, giving the gem a dynamic and captivating appearance.
Labradorite is often used in jewelry and is highly prized for its unique play of color.