Apatite as a gemstone

A crystal of gemmy apatite on a matrix of quartz.
Photo by Rob Lavinsky

Apatite is a whole family of minerals some of them are gems, but the place where you are most familiar with is your teeth and skeleton that are composed of hydroxalapatite.  Basically apatite is composed of calcium phosphate with extra ions added to its crystalline structure to cause it to be known as several related names: flourapatite, chlorapatite, bromapatite and in the case of your bones the OH ion. Apatite is used as the defining mineral for number five on the Moh’s hardness scale. Hydroxyapatite is a relatively rare form of the mineral are mostly groups are absent or contain many carbonate and acid phosphate substitutes that form a large part of bone material.

A twinned crystal of Apatite
Photo by Rob Lavinsky

Flourapatite is much more resistant to acid attack than hydroxyapatite it is for this reason that fluorine is added to your drinking water and can also be found in your toothpaste. In the case of the fluoridated water it allows the interchange of fluoride ions that substitute for hydroxyapatite ions. If you get too much fluoride into your system it can result in dental or skeletal flouresis.

Fission tracks are used by geochemists and geophysicists for determining the thermal history of orogenic belts and settlements in sedimentary basins. This is because as a trace element uranium and thorium are found in crystalline apatite. The same phenomenon is also used in paleontology to establish the dates of prehistoric wildfires.

A faceted gemstone of apatite.
Photo by Rob Lavinsky

Apatite in phosphorite is a phosphorus rich sedimentary rock containing between 18 and 40% of phosphorus pentoxide (P₂O₅). A great deal of the phosphorite is mined in central Florida in an area that is called “Bone Valley” for use as fertilizer. It contains so much uranium that during the Cold War if supplied several thousand tons of yellowcake uranium to the Atomic Energy Commission. The piles of mine slimes resulting from mining and refining this phosphorite presents a serious environmental hazard not only from the uranium, but also from the contained fluorides. The apatite contained in phosphorite is in crypto- crystalline masses that are called colophane. Apatite is also mined from igneous rocks in the Kola Peninsula of northeast Russia as a fertilizer as well..

Clear crystals of apatite can be faceted creating an extremely handsome gem. Because of the sensitivity to heat and shock gems that are cut from apatite are never heat-treated to improve their looks. Although apatite is found all over the world most of it is small crystals disseminated through the rocks; larger crystals are quite rare. Most of the faceting grade apatite crystals are shades of blue, although some are blue-green.

A thin section of hornblende and apatite
Photo by Pitor Sosnowski

Most of this faceting material is found in metamorphosed limestone that is commonly called marble. In general the more impurities that are found in limestone the more apt it is to produce apatite when it is metamorphosed. There is a bed of metamorphosed limestone that reaches all the way from Québec to Alabama just of the West of the Precambrian core of the Appalachians. In many places this is called the Great Valley of the Appalachians but the same marble belt extends all the way to Canada. There is apatite found throughout this entire region, and others like it all over the world.