Fluorite

Pretty much every mineral collection includes a fluorite example.  It's a common, beautiful, and is second only to quartz in popularity.  Fluorite fractures (called cleavage) on four planes so this octahedron shape is a common form for the mineral.  Pure fluorite is clear but, because of impurities, it has a large color range.  In addition, a fluorite specimen might have a different color under shortwave UV light than longwave UV light, and phosphorescence in yet a different color.  Some crystals will thermoluminescence too (means when it's heated it will glow). While fluorite has many industrial uses, it's hardness is only 4.  You'll see it in those mineral collections as a crystal like this or even faceted but rarely used as a gemstone.