All things are made of atoms. In the center of every atom there is a nucleus (the core). Around these nucleus, electrons move on diffirent electronshields.

These electrons turn around the nucleus of the atom. This balance can be changed by energy changes. One of these possibilities is getting hit by UV-light.

When there is a high energy source (UV light, excisting out of fotons) hitting these electrons, they come out of balance and are pushed towards a higher electronshield. Because of other electrons on this shield, it is impossible to stay there, so the electron will return to his original shield. During this 'jumping back' to his shield, it released energy as light. This is the fluorescent color of the stone we can see.