Fluorite

Found all over the world, the name "fluorite" comes from the Latin "Fluo" - meaning "to flow." Fluorite has traditionally been used as a flux - added to other substances, i.e. steel, to make it flow better and to remove impurities.
Fluorite will also fluoresce in ultra-violet light - and is the origin of that term - fluoresce, fluorescent, etc., are named for this interesting and colourful mineral - not the other way around.
It has a hardness of 4 on the Mohs scale - which is not particularly hard.