What is Fluorescent?
If you turn on a black-light bulb in a dark room, what you can see from the bulb is a purplish glow. What you cannot see is the ultraviolet light that the bulb is also producing.
Our eyes can see visible light in a spectrum ranging of red through orange, yellow, green, blue and violet. Above violet is ultraviolet light, which we cannot see. What you see glowing under a black light, whether on a fluorescent painting or mineral-stone are phosphors.
A phosphor is any substance that emits visible light in response to some sort of radiation. A phosphor converts the energy in the UV radiation from a blacklight into visible light.
Phosphor are all around us, white T-shirts and socks normally glow under a black light because modern detergents contain phosphors that convert UV light into white light. This makes whites look “whiter than white” in normal sunlight. What you are seeing in sunlight is the normal reflection of visible white light from the cloth, as well as the emission of white light that the phosphors create from UV light in sunlight. The T-shirt really is whiter than white.
All mineral-stones are more or less fluorescent.
Below is an good example of the mineral-stone Calcieten without and with Blacklight.