Synthetic cubic zirconia

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From The Gemology Project
Synthetic cubic zirconia
Chemical composition Zirconium dioxide - ZrO2
Crystal system Cubic
Habit Columnar
Cleavage
Fracture
Hardness 8.25 - 8.5
Optic nature Isotropic
Refractive index 2.171 - 2.177
Birefringence None
Dispersion High, 0.059 - 0.065
Specific gravity 5.65 - 5.95
Lustre
Pleochroism None

Synthetic cubic zirconia (CZ) entered the market as a gemstone around 1976 and is till present 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.

Contents

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 can not 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.

References



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