Apatite: Gemstone Information
Once assigned a group of their own the apatite minerals chlorapatite,fluorapatite and hydroxylapatite stand alone as independent species.They are monoclinic or hexagonal phosphates. Gem material apatites are usually hydroxylapatite or fluorapatite; the fine transparent yellow Mexican material is hydroxylapatite and the purple material from Maine is fluorapatite. For the gemmologist, there is no need to make miner alogical distinctions providing that an apparent apatite does not prove to be something quite different. Apatite is found in a wide range of situations; gem apatite usually occurring in pegmatites, hydrothermal veins or alluvially.Apatite can be found in a wide range of colours with violet (especially from Maine) being notably attractive. The Mexican yellow apatite is famil-iar to students for its rare earth fine line absorption spectrum and a rather beautiful quiet green has been in and out of the connoisseurs’ market in the past few years. A vibrant blue, resembling and rivalling the bright copper blue of the Paraíba tourmaline, has been found in Madagascar.Apatite’s softness at 5 rules out hard wear: the hexagonal crystals are often quite large.
The SG falls in the range 3.17–3.23 with some blue specimens near 3.18 and yellow ones 3.22. RIs are in the range 1.63–1.64 uniaxial negative with birefringence low at 0.002–0.004. Dichroism is not usually prominent though distinct in the blue material. Both blue and yellow apatites may show the fine-line RE absorption spectrum in which elements of both neodymium and praseodymium are combined. Yellow apatite’s absorption spectrum is characteristic, consisting of two groups of fine lines, one group in the yellow, the other in the green. Blue apatite shows broader elements with strong bands at 512, 491 and 464 nm.The apatite minerals respond variously to UV with some yellow speci-mens showing a lilac-pink under both LW and SW and some blue stones giving bright, deep violet-blue to sky blue. There would be little need for evaluating apatite’s response to X-rays. Beautiful blue stones are found in the Mogok Stone Tract of Myanmar and in the gem gravels of Sri Lanka, some of which have a fibrous structure making a cat’s-eye possible. Yellowish green crystals found in the Spanish province of Murcia, probably in the Serra de Espuna, have been called ‘asparagus stones’, from their characteristic colour. From Arendal, in southern Norway, comes a beautiful bluish green variety. Well-formed prismatic crystals of yellow colour come from Cerro de Mercado, Durango, Mexico.
Very fine violet-coloured crystals are found at the Pulsifer Quarry,Androscoggin County, Maine, USA, one of the great mineral localities.A deep rich green apatite is found in Quebec and Ontario, Canada, andmarketed as ‘trilliumite’. There are many other sources of apatite, which is a widespread mineral, but the localities given are the better-known places where gem material is found. Well-formed green crystals in an orange calcite come from Mozambique and form fine mineral specimens, the apatite often being clear enough to produce cut stones. Yellow and green cat’s-eye apatite are not uncommon and an intense sapphire-blue material comes from Minas Gerais, Brazil. Yellowish green, greenish brown and reddish brown apatites and apatite cat’s-eyes have been found in the Umba valley in Tanzania. Green apatites from Rössing in Namibia have high RIs of 1.649–1.655 with a birefringence of 0.006. A massive sky-blue variety of apatite has been polished as an orna-mental stone, and a variety of lapis lazuli from Siberia has been named lazurapatite.