To See Schiller You Need Light
Schiller which comes from the German word for 'twinkle' is an optical phenomenon that can be seen in certain minerals and gemstones. Caused as light reflects off layers beneath the surface of the stone, the iridescent sheen that we see is caused by the reflection of light. As with all optical phenomena without the presence of light they would't be visible.
When schiller is present in the mineral labradorite it's often referred to as labradorescence. This term came about because labradorite is renowned for this optical phenomenon. The word adularescence is also sometimes used but tends to be associated with the subtle glow of light that appears across the surface of moonstone. The word opalescence is also sometimes used.
In moonstone that's more opaque exceptional schiller can often be seen. Stone's that exhibit this vibrant flash of blue colour tend to be known as rainbow moonstone.