Apatite

With a value of only 5 on the Mohs scale of hardness, apatite is seldom faceted as a gemstone, except for collectors. However, when cut correctly, stones are bright with strong colours. Transparent to opaque, apatite occurs as colourless, yellow, blue, violet or green hexagonal prisms or tabular crystals.
Apatite is an abundant mineral, found in many types of rock, but most gem-quality material is associated with pegmatites. Blue Burmese apatite is strongly dichroic, and shows colourless or blue when viewed from different directions. Fibrous blue apatite from Burma and Sri Lanka may be cut “en cabochon” to show a cat’s –eye. Chatoyant stones are also found in Brazil, along with yellow, blue and green varieties. Other localities include the Kola Peninsula (Russia), Canada, East Africa, Sweden, Spain and Mexico.
Spanish apatite is often called “asparagus stone”, due to its yellowish green colour.

Properties

Chemical Composition: Calcium phosphate with some chlorine or fluorine. Ca(F,Cl)Ca₄(PO₄)₃

Classification / Type: -

Colors / Varieties: All colors. Chatoyant variety is very common.

Crystal System / Forms: Hexagonal System / Generally as six sided prisms with pyramidal and / or pinacoidal terminations, commonly as contact twins.

Hardness: 5

Specific Gravity: 3.15 - 3.23

Cleavage / Fracture: 2 directional basal and prismatic cleavage / conchoidal fracture

Optic Character: Anisotropic, D.R.; Uniaxial negative (pseudo biaxial optic figure possible).

Lustre: Vitreous.

Refractive Index / Birefringence: 

1.642 - 1.646 / 0.002 - 0.006. Range:1.637 - 1.651

Pleochroism: Weak, except in Burmese (Myanmar) gemstones.

Dispersion: 0.013

Magnification: Fibrous inclusions, parallel black needles or canals, liquid fingerprints, phase inclusions and crystals.

U.V. Fluorescence: Varies according to color under ultra violet light.

Spectrum: Generally, a series of lines at around 580nm and 520nm in the yellow-green, with other lines at 512nm, 491nm and 464nm may be seen. Rare earth didymium spectrum composed of mainly the elements praseodymium and neodymium. Blue apatite may not show spectrum, sometimes.

Cause of Color: 

• Yellow green - rare earth elements
• Blue - manganese.

Treatment (Enhancement): -

Specific Tests & Remarks: 

• Attacked by hydrochloric and sulphuric acid.
• Reacts to heat and may lose color.

Synthesis: Synthesized for use in lasers but cut synthetics are not sold commercially.

Simulants (with key separation tests): 

• Transparent varieties can simulate a number of different gemstones with respect to their color.
• Apatite cat's eye: Quartz (R.I., S.G., spectrum, inclusions), Chrysoberyl (R.I., S.G., spectrum, inclusions), Glass (inclusions), etc.

Geological Occurrence: Found in pegmatite, hydrothermal veins and cavities, metamorphic rocks in gem gravels.

Sources: Sri Lanka, Myanmar, Norway, Canada, India, Germany, Pakistan, Madagascar, Tanzania, Brazil.

Cuts & Uses: Facetted cuts, cabochon, beads, carvings, etc.