Final answer:
Asterism reveals a star-like pattern, chatoyancy gives a 'cat's eye' effect, iridescence causes color variations with changes in view or light, and labradorescence is a type of iridescence causing brilliant color flashes.
Step-by-step explanation:
The optical effects of asterism, chatoyancy, iridescence, and labradorescence are fascinating phenomena that result from the interaction of light with the microstructure of gems and other materials. Asterism is an effect where a star-like pattern appears on the surface of a gemstone when it is cut en cabochon. This happens due to light reflecting off of inclusions within the stone, such as needle-like crystals, that are arranged in certain patterns.
Chatoyancy, also known as the "cat's eye" effect, is seen in some gemstones that have fine parallel inclusions, creating a single bright band of light across the stone that resembles a cat's eye. This effect can be seen when the stones are cut with the inclusions parallel to the base of a cabochon.
Iridescence is the phenomenon where an object appears to change color as the angle of view or angle of illumination changes. This effect can be observed in soap bubbles and butterfly wings. It is caused by the interference of light rays reflecting off of the microstructural layers of the object.
Labradorescence is a specific type of iridescence that occurs in labradorite, a type of feldspar that shows an impressive display of colors due to light interference within its layers. It exhibits brilliant flashes of color when light strikes it from particular angles.