Colors of fluorite

The mineral fluorite consists of calcium fluoride (CaF2). It can have variation in color, largely due to impurities in its crystal structure. Flourite can be found throughout the world. It is common in hydrothermal deposits, where it can be associated with quartz, calcite, baryte, and galena (lead sulfide). 

Uses of fluorite include jewelry making, as well as a flux for smelting. 

The largest crystal (green color one) is 4 cm long.

Flourite can crystallize in several forms, including the octahedral form, which is my favorite.

Octathedral crystals: the clear one is 3 cm tall.

Flourite has a hardness of four on the Mohs Scale. Optically clear crystals, like the one shown above, has low aberration, thereby making them valuable in the construction of microscopes and telescopes.