Apatite

Apatite · also called Fluorapatite, Asparagus stone, Moroxite

Apatite is a calcium phosphate, Ca₅(PO₄)₃(F,Cl,OH), and the mineral that defines point 5 on the Mohs scale. It is cut for colours ranging from a saturated blue-green associated with Madagascar to the yellow-green of Mexican material, and it is soft enough that both setting and cleaning need deciding in advance.

What is apatite?

Apatite is not a single mineral but a small group of calcium phosphates sharing the formula Ca₅(PO₄)₃(F,Cl,OH), the members separated by whether fluorine, chlorine or hydroxyl occupies one shared site. Fluorapatite is the member almost always met as a cut gem.

The group is among the most abundant phosphate families in the crust and the principal ore of phosphorus, mined in enormous tonnage for fertiliser. It is also the mineral of the body: tooth enamel and the mineral fraction of bone are a carbonated hydroxylapatite, which makes apatite the one gem species a buyer is already carrying in their own jaw. Transparent crystals clean enough to facet are a minute fraction of what is taken out of the ground, and they come from granitic pegmatites, from metamorphosed limestones, from alkaline igneous complexes and from the alluvial gravels that drain all three. Apatite is also the reference mineral for point 5 on the Mohs scale, a fact that follows the stone through every conversation about wearing it.

What gives apatite its colour?

Apatite occurs in nearly every colour. Manganese, iron and rare-earth elements substituting into the calcium site are the principal causes, and because the trace chemistry differs from deposit to deposit, colour is a rough guide to origin. Blue-green, yellow-green, violet, pink and colourless material all occur naturally.

The colour that carries the modern trade is a saturated blue to blue-green, sometimes strong enough to read as electric under daylight, associated above all with Madagascar. It is routinely marketed by comparison with cuprian tourmaline, and the comparison is a description of appearance only: the colour of Paraiba-type tourmaline is caused by copper, and apatite's is not. Yellow to yellow-green material, the "asparagus stone" of the older literature, is the other historically important colour and is associated with Mexican production. Violet and purple crystals occur but are usually small and are more often kept as specimens than cut. Pink apatite is scarce and typically included. Colourless material is faceted as a curiosity rather than as a commercial line.

How is apatite identified?

A refractive index between about 1.628 and 1.649 with a very low birefringence of 0.002 to 0.008, a uniaxial negative optic figure, and a specific gravity of 3.16 to 3.23 settle the identification quickly. Dispersion is 0.013, low. A hardness of only 5 is corroborating evidence in itself.

Those readings do the separating that colour cannot. Blue apatite is confused most often with blue tourmaline, which reads near 1.62 to 1.64 with a birefringence around 0.018, high enough that facet edges double visibly through a loupe. Aquamarine reads lower at 1.577 to 1.583 and is noticeably lighter in the hand at a specific gravity near 2.7. Blue zircon is not a real candidate once an instrument is involved, reading above 1.92 with a birefringence near 0.059 and a specific gravity above 4.6. Green apatite is separated from peridot, which reads 1.654 to 1.690 with obvious doubling. Yellow apatite is separated from chrysoberyl at 1.746 to 1.755. Where no refractometer is available, a hardness point and a specific gravity determination together are close to conclusive, and the absorption spectrum described below is better still.

The distinguishing feature: a rare-earth absorption spectrum

Yellow and yellow-green apatite shows one of the most distinctive absorption spectra in gemmology: a group of fine, sharp lines produced by the rare-earth elements neodymium and praseodymium, the pair the older literature called didymium. No other yellow gem in ordinary circulation produces that pattern.

Seen through a hand spectroscope with a strong light behind the stone, the lines appear as a tight group rather than as the broad bands most coloured stones give, with further fine lines elsewhere in the spectrum. Blue and blue-green apatite tends instead to show broader absorption and is less immediately conclusive. The logic of the test is worth stating plainly: the presence of the rare-earth lines confirms apatite, while their absence does not rule it out. Taken with a hardness of 5 and the refractive index, the spectrum makes apatite one of the more comfortable identifications in the yellow-to-green range.

Is apatite treated?

Most apatite reaches the market untreated. Heating is reported for part of the blue and blue-green production, to remove a greenish or brownish component, and surface-reaching fractures are sometimes filled with a resin or oil to make them less visible. Dyeing is confined to opaque and heavily fractured material.

Heat, where it has been applied, is stable and leaves no residue that shows on inspection, which is why it is disclosed by declaration rather than found in the stone. Filling is a different matter and can be found: turn the stone under a bright light with a loupe and a filled break flashes a colour that the surrounding stone does not have, and at the boundary the fracture surface looks smoothed rather than glassy. Because apatite is brittle and soft, breaks reaching the surface are common in cut material, so this is a check worth making on every stone rather than only on suspect ones. Ask for treatment status in writing, and for stones of consequence ask for a report from a recognised laboratory; the treatment vocabulary used by GIA and the LMHC member laboratories is the standard the trade is held to.

Where does apatite come from?

Gem apatite is widely distributed. Madagascar supplies most of the strong blue to blue-green material, Mexico the classic yellow-green, and Brazil a broad range of blue, green and yellow. Myanmar, Sri Lanka, East Africa, Canada, Portugal, Russia and Maine all contribute.

Madagascar's blue-green production, largely from the south of the island, is what made apatite a stone the coloured-stone trade takes seriously rather than a collector's oddity. Mexican material from the iron deposit at Cerro de Mercado, near Durango, is the reference yellow-green and has been cut for well over a century. Brazilian material comes chiefly from the pegmatite belts of Minas Gerais. Myanmar and Sri Lanka yield both faceting rough and the fibre-included crystals cut as cat's eyes, as does Madagascar. East African production from Kenya, Tanzania and Mozambique covers greens and blue-greens. Canadian crystals from Ontario and Quebec are large and yellow-green, more often kept as specimens than cut. Portugal and the pegmatite quarries of Maine supply the violet and purple crystals collectors pursue. Russia's Kola Peninsula deposits are vast, but that is phosphate mining rather than gem mining, and it is a reminder of how little of this mineral is ever cut.

Where the name comes from

Apatite was named in 1786 by the German mineralogist Abraham Gottlob Werner, from the Greek verb meaning to deceive, because the material had repeatedly been taken for other species: beryl, peridot, amethyst and tourmaline among them. The name is a warning label, and it has aged well.

Two older varietal names still surface in catalogues and old collections. Asparagus stone was used for the yellow-green material, after the vegetable it resembles in colour. Moroxite was used for dull blue-green crystals of Scandinavian origin. Neither term carries any modern gemmological meaning, and neither should appear on an invoice as though it were a grade. Contemporary marketing has added its own vocabulary, and terms such as neon apatite describe an appearance rather than any defined category.

What sizes are available?

Availability depends heavily on colour. Yellow-green material from Durango is cut clean well into double figures of carats without difficulty. Strong blue and blue-green material is generally cut small, and clean stones of more than a few carats are markedly less common than the marketing around the colour suggests.

Two things work against large cut stones. The rough is brittle and parts readily under the saw and on the lap, so cutters take conservative yields and lose material at every stage. And the strongest colours often sit in rough that also carries the most included, most fractured zones, so the choice is repeatedly between size and cleanliness. The practical consequence is a price curve that is flat for the pale and the yellow-green, where size is not scarce, and steep for saturated blue-green, where each additional clean carat is harder to find than the last. Cat's eye material is cut as cabochons of modest size, since the fibre inclusions that give the eye also limit how large a clean dome can be taken.

How much is apatite worth?

Colour dominates, then clarity, then size, then cut. Saturated blue-green material sells for a multiple of any other colour in the species. In absolute terms apatite remains an inexpensive gem: even the finest neon-coloured stones sit far below tourmaline of comparable colour and size.

The reason for that ceiling is not appearance but wearability. A hardness of 5 keeps the stone out of the categories that carry the highest prices, because the jewellery trade prices durability as well as beauty. Within the species, the ranking is straightforward. Strength and purity of blue-green come first, and a grey or brownish modifier costs more than any other single fault. Eye-visible inclusions matter next, particularly reflective fractures, which read as bright flaws in a stone this transparent. Size counts only after those two. Cutting quality is the most commonly neglected factor: because the material is soft, a lot of apatite is cut quickly and shows abraded facet junctions and a dull polish, and a well-cut stone with a crisp polish looks conspicuously better than the price difference implies.

Is apatite durable enough to wear?

Apatite is soft and brittle. Hardness is 5, below the hardness of ordinary airborne dust, so an exposed surface abrades with wear rather than resisting it. Cleavage is indistinct and basal, the fracture is conchoidal, and the material is attacked by acids.

That combination sets the terms. Earrings, pendants and brooches are the natural homes for apatite, where the stone is not struck against surfaces. Rings are possible only in protective settings, with a bezel or a recessed design, and only for occasional wear. Clean with lukewarm soapy water and a soft brush, and nothing else: ultrasonic cleaners and steam are both unsuitable, the first because vibration finds existing fractures and the second because sudden heat does. Household cleaners containing acid will etch the surface, as will the acidic residues of some cosmetics over time. Stones with filled fractures need more care again, since solvents and heat can disturb the filler. None of this makes apatite unwearable; it makes it a stone that survives if the setting is chosen honestly at the time of purchase.

How to buy apatite without being caught out

The commonest problems are language rather than substance. Comparisons to Paraiba are marketing about colour, not a statement of relationship, since that term belongs to copper-bearing tourmaline and is defined by the LMHC for that material. Read every varietal or trade name as description until proved otherwise.

A short checklist covers the rest. Ask directly whether the stone has been heated and whether any fracture is filled, and have the answer written on the invoice. Loupe the stone under a bright light and turn it, looking for the flash of a filled break and for abraded facet junctions, both of which are common in this species. Check the girdle for chips, which brittleness makes likely and which a bright setting will hide. Judge the colour under two light sources and look for a grey modifier, which is the fault most often disguised by photography. Confirm the seller states the hardness, because a seller who does not mention it is either careless or is selling on appearance alone. Finally, decide the setting before the stone: an apatite bought for a daily-wear ring is a different purchase from the same stone bought for a pendant, and the advice we give starts from what the stone will be asked to survive. Comparable material is discussed across the wider coloured stone pages.

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