Malachite and azurite are formed in the same way, but azurite is a bright blue and is the rarer of the two minerals.
Sources are Arizona, Africa, Australia, and France.
Optical calcite has a special property that produces a double image when you look through the mineral. This double image is produced as light reflects off the atoms in the mineral's structure, splitting into two rays. Flourite is a mineral that combines calcium and fluoride. It forms crystals shaped like cubes within limestone. Flourite is associated with lead ores and is one of the most popular minerals among collectors.