So why exactly do gems change color under a blacklight? Is it both calcite and fluorite that cause this? Are there any additional chemical elements in minerals or gems that cause them to change color?
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Gem-A has a great video about this: Gem-A Live: Fluorescence - YouTube
And an article: Focus on Gemstone Fluorescence: Looking for the Light
You’re blacklight has a different wavelength that reacts with the chemicals in rocks and minerals that creates fluorescence. Rocks reflect light, that’s how we can see them! Some though, that have fluorescence, can absorb a tiny amount of light and then release some of that light in a different wavelength. This change can cause the color change when looked at with a black light.
Some can be very stunning. One example is “Yooperlite” not really a real rock name but more of a trademarked name given to a stone found along the shores of Lake Superior in Northern Michigan. This rock, in daylight, is seen to be just another boring grey looking rock amongst billions of other, boring grey rocks along the shoreline. But…go out at night with a UV light and these stones light up! They are a syenite rock that has a rich amount of fluorescent sodalite in them to make them literally glow in UV light.