Blue and yellow apatites display a rare earth (“didymium,” i.e. praseodymium + neodymium) spectrum. Yellow gems have 7-line group at 5800 and 5 lines at 5200. Blue gems give broad bands at 5120, 4910, and 4640.
Very variable with composition:e = 1.598-1.666; o = 1.603-1.667. Gem varieties: o = 1.632-1.649, e = 1.628-1.642. See "Identifying Characteristics" below. Uniaxial (-); francolite may be biaxial, 2V = 25-40°.
Uniaxial -, Biaxial -
From the Greek apatein, meaning “to deceive,” because mineralogists had confused apatite with other species.
Apatite is found in a wide variety of rock types. Igneous rocks are usually characterized by F and OH varieties, some containing Mn. Apatite occurs in pegmatites, hydrothermal veins and cavities, metamorphic rocks, and as detrital grains in sedimentary rocks and phosphate beds.
The apatite mineral group occurs abundantly throughout the world. In fact, bones and teeth consist mainly of a variety known as hydroxylapatite. As collophane, a massive cryptocrystalline variety that occurs in some localities as large beds, apatite also serves as the most abundant phosphorus-bearing mineral.
Most of the gemstones referred to as apatite are of the commonest variety, fluorapatite. Other gem-quality apatites include the aforementioned hydroxylapatite as well as carbonate-rich apatite, mimetite, moroxite, vanadinite, wilkeite, and the rare chlorapatite. Lazurapatite, a mixture of lapis lazuli and apatite, occurs in Siberia.
Speaking strictly mineralogically, the term “apatite” refers to the mineral group only. However, you’ll find many gems commonly described just as apatites, without further classification.
Prior to the discovery of the now coveted neon blue-green apatites in Madagascar, these colorful but little-known gemstones seldom found their way into jewelry collections. Nevertheless, mineral collectors prized them not only for their colors but also their beautiful fluorescence.
Since the Madagascar finds, the jewelry use of apatites of all colors has increased. Although the color of the best neon-blue green specimens can approximate that of the storied Paraíba tourmalines, all gem-quality apatites have a hardness of 5 at best. Alas, apatites don’t have the toughness of their tourmaline rivals. They require special care as jewelry stones.
Mexican Yellow Apatite
Mexican yellow apatite is perhaps the most abundant gem material available. Thousands of crystals exist that would cut stones up to five carats.
Chatoyancy can occur in blue-green and green gems from Myanmar and Sri Lanka. Brazil can produce green cat’s eyes. Sri Lanka and Tanzania can produce yellow cat’s eyes. The chatoyancy in Tanzanian apatites can be so intense that the material resembles cat’s eye chrysoberyl.
Blue gems (Brazil) almost always range on the small side (1-2 carats). Myanmar produces 10-carat blue gems. However, this color is very scarce in larger sizes.
Yellow gems from Mexico up to 15-20 carats are known, but larger ones are quite rare.
The rarest and smallest in general, violet stones usually fall under 2 carats. However, the Roebling purple apatite in the Smithsonian Institution weighs approximately 100 grams (500 carats), a superb crystal.
Blue-green clean stones usually weigh less than 5 carats, very rarely more.
Green apatite occurs in large crystals. Canadian material has yielded 100-carat flawless stones. The world’s largest golden green gem may be a 147-carat stone from Kenya.
Yellowish cat’s eyes range up to about 15 carats, while green cat’s eyes can weigh a bit larger (20 carats).