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16.3.2: Sapphire

  • Page ID
    4026
  • Sapphire
    Chemical composition Al2O3
    Crystal system Trigonal
    Habit Prismatic, tabular
    Cleavage None, may show parting on twinned stones
    Fracture Conchoidal
    Hardness 9
    Optic nature Uniaxial -
    Refractive index 1.762-1.770
    (+.009,-.005)
    Birefringence 0.008-0.010
    Dispersion Low, 0.018
    Specific gravity 4
    Lustre Vitreous
    Pleochroism Moderate to Strong

    File:Ceylon.JPG

    Figure \(\PageIndex{1}\): Cornflower Ceylon sapphire

    Sapphire image gallery

    Sapphire (a corundum variety) is an aluminum oxide occurring in every color of the rainbow. It is a stone of great hardness and durability. It can also have phenomenal characteristics like asterism (star sapphire) and color changing (like alexandrite). The color changing varieties are mesmerizing, having the ability to change color depending on whether they are viewed in daylight or incandescent light.

    The name is derived from the Greek word "sappheiros" meaning "blue". The history of sapphire dates back to at least the 7th century BC, when they were used by the Etruscans. The sapphires used by the Etruscans, Greeks, and Romans were imported from India (what is now Sri Lanka). Sapphires were reputed to protect kings from harm and envy. In the 13th century, it was written that sapphires had the power to protect against poverty, to make a stupid man wise and an irritable man good-tempered.

    Diagnostics

    Color

    Sapphire occurs in many colors ranging from colorless to black. When a corundum variety is termed "sapphire" it indicates the blue variety. Any other color (except red, which is named ruby) will have a prefix before "sapphire", such as "yellow sapphire".

    Causes of color:

    • sapphire (blue) - charge transfer between Ti and Fe.

    Diaphaneity

    Transparent to opaque

    Refractometer

    Sapphire has a refractive index range between nω = 1.767-1.772 and nε = 1.759-1.763, with a maximum birefringence of 0.009.
    The optic sign is negative like most, naturally occurring, uniaxial gemstones.

    Specific gravity

    The specific gravity of sapphire is between 3.98 and 4.02 (mean = 4).
    It will sink in all common used heavy liquids.

    Spectrum

    Figure \(\PageIndex{2}\): Spectrum of blue, green and Australian yellow sapphire (high iron content)

    Natural blue, green and yellow sapphires with a high iron content may show the typical "450 complex" as seen in this image. The clear lines at 450 and 460 nm (less sharp than the 450 nm line) will be accompanied with a third (sometimes faint) line at 470 nm. Due to partial absorption of wavelengths between these 450 and 460 nm lines, this whole section may blend together with "smudges" between the lines.
    With lesser iron content only the 450 nm line may be observed in natural blue and yellow sapphire. Although this same 450 nm line can also be observed in some blue flame fusion (Verneuil) synthetic sapphire, the "450 complex" has not been reported for synthetic sapphire.

    Polariscope

    Most natural sapphires are cut with the table almost perpendicular to the optic axis and a uniaxial interference figure should be easily found. Some synthetics (Verneuil type) are cut with the table parallel to the optic axis and the interference figure will be hard to find on the girdle.
    Finding an interference figure on the table is not diagnostic.

    Phenomena

    Asterism

    Sapphire may show 6 pointed or 12 pointed stars.
    The 6 pointed rays are from reflections on rutile needles that form in directions parallel to the 2nd order prism. 12 pointed rays form from reflections on rutile needles (2nd order prism) and from reflections of hematite-ilmenite needles that lie in the planes parallel to the 1st order prism.

    Color change

    Cat's-eyes

    Trapiche

    Synthetics

    Flame fusion (Verneuil)

    Color change flame fusion sapphire will have a characteristic absorption spectrum with a fuzzy band in the yellow and a diagnostic line at 473 nm (in the blue, called the vanadium line) In some rare cases this 473 line may be seen in natural sapphire.
    Curved growth lines are usually seen.

    Other synthesizing methods

    • Flux melt (Chatham, Ramaura)
    • Czochralski pulling process
    • Float zone method

    Occurrence

    Kashmir, Burma, Sri Lanka, Madagascar, Australia, China, Montana USA, Thailand

    Enhancements

    Common enhancements to sapphire varieties:

    • Fracture filling - oil, wax or plastic (with dye or without)
    • Lead glass filling - removes crack increases weight
    • Diffusion treated - heating causing layer on cut stones - enhance color or produce asterism
    • Heat treatment - improves or even changes the color, reduces silk and other impurities
    • Irradiation - change colorless stones to yellow - color unstable
    • Beryllium treatment - Heated with beryllium as an Additive to produce Orange/yellow/blue colors

    A short Film on the Sapphire Mining Techniques in Bang Kacha

    Bang Kacha, in Thailand, produces predominately green and golden-orange colored sapphires.
    Small quantities of blue sapphires, as well as some star stones, are found here.

    A short Film on Sapphire Mining in Ilikaka, Madagascar

    A short Film on Sapphire Processing in Sri lanka ( ceylon )

    This film Shows a trip to the mines of Ratnapura, Pale sapphires of blue and yellow are sorted and heat treated at 1000 C for a few hours to produce White Sapphires. The heat dissolves all silk and Makes the stone whiter and brighter to use as a natural substitute for diamonds.

    Sources

    • A students' guide to spectroscopy (2003) - Colin H. Winter
    • Ruby & Sapphire (1997) - Richard W. Hughes ISBN 0964509768