Apatite has a Hexagonal crystal system and is in the mineral class of phosphate. Its coloring can range from being grey, yellow, green or blue. The most common coloring we are used to seeing in jewelry or in its tumbled varieties would seem to be the bluer aqua shades.
Apatite is generally fairly opaque in its appearance and rarely found completely transparent. Its origins as a crystalline phosphate have generally occurred from magmatic activity. While more rare, it can have a sedimentary origin which seems to produce the more visibly transparent varieties. It is found over much of North America, Mexico, Norway, Russia and Sri Lanka.