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14.7: Synthetic Cubic Zirconia

  • Page ID
    3819
  • Synthetic cubic zirconia (CZ) entered the market as a gemstone around 1976 and is still presently the most widely used diamond simulant. Although cubic zirconia is found in nature (as minute crystals inside zircon), the more stable variant of zirconia crystallizes in the monoclinic system (baddeleyite).
    The main means of syntheses is through the "skull crucible" aka "skull melting" method.

    Chemical composition

    Zirconia has the chemical formula ZrO2, this material, however, will not crystallize in the cubic system through syntheses. A stabilizer of yttria or calcium oxide needs to be added to the source powder in order to let it crystallize as isotropic crystals.

    Diagnostics

    As synthetic cubic zirconia (CZ) is mainly used to imitate diamond one can expect the main separation techniques to concentrate on distinguishing between the two.

    Color

    Synthetic cubic zirconia is produced in a large array of colors, from colorless to black. Many coloring agents, including rare earth elements, are added to the source powder.

    • Cesium: yellow, orange, red
    • Copper, iron, nickel, praseodymium, titanium: yellow, amber, brown
    • Erbium, europium, holmium: pink
    • Chromium, thulium, vanadium: green
    • Cobalt, manganese, neodymium: lilac, violet, purple, blue

    Diaphaneity

    Transparent to opaque.

    Refractometer

    Cubic zirconia has an RI that cannot be measured with a standard gemological refractometer. There are however other tools available, as the Hanneman-Hodgkinson refractometer or the Brewster angle meter, to determine the RI of CZ.

    The optical properties of CZ vary slightly depending on which, and how much, stabilizer is used (yttria or calcium oxide).

    • Yttria stabilized cubic zirconia: RI = 2.171, dispersion = 0.059
    • Calcium oxide stabilized cubic zirconia: RI = 2.177, dispersion = 0.065.

    The values of CZ are above the limits of the standard gemological refractometer. There are however a few other tests one can perform.

    • The "dot test" will show a ring around the culet if the stone is cut to diamond brilliant proportions.
    • The "tilt test" will show leakage.

    Polariscope

    CZ may show anomalous extinction.

    Specific gravity

    The SG of CZ is about twice that of diamond (3.52) and loose stones will have a higher "heft".
    As with the refraction indices, the SG of CZ varies between stabilizers used.

    • Yttria-stabilized cubic zirconia: SG = 5.95
    • Calcium oxide stabilized cubic zirconia: SG = 5.65

    Hardness

    • Yttria-stabilized cubic zirconia: hardness = 8.25 (Mohs' scale)
    • Calcium oxide stabilized cubic zirconia: hardness = 8.5 (Mohs' scale)

    Luminescence

    Colorless CZ stabilized by calcium oxide will show a yellow fluorescence.

    Phenomena

    Color change synthetic cubic zirconia is not uncommon and may show dramatic changes in natural/incandescent lightning.
    Opalline (milky) CZ's are also produced. These are mainly opaque.