Welcome... to a rather misunderstood topic.
If you want your engagement ring to look amazing, it's important to ensure the main diamond is the star of the show and has all the brilliance and fire you would expect from a diamond.
Fluorescence is a little-known diamond specification that can drastically alter the price and appearance of any diamond you cross paths with.
Mother nature rolled the dice and around 30% of diamonds emit some degree of a fluorescent glow when exposed to concentrated UV light. It's random, as you can see in the image above.
As you know there’s UV in sunlight, and although far from concentrated there is a small reaction a diamond has with the UV light in natural sunlight.