Fluorite

Otherwise known by Fluorspar or Fluoride

Fluorite is an interesting material. It produces some of the most visually appealing mineral specimens in nature, with a huge variety of colors and habits. Add on it's weak refractive index, and it has made for very visually interesting cubic specimens.

Industrially, it's rather unique. It has optical qualities that make it exceptionally attractive for use. It allows a large range of wavelengths to pass through without issues. Still, it is among the rarer synthetics out there. It's quite uncommon to come by lenses made out of it, let alone uncut boules. Sadly it's not always preferred in some situations because fluoride lenses still suffer from the softness, brittleness, and thermal sensitivity of natural fluorite. This makes it less optimal for situations where tougher lenses are required for the environment that they're being used in.