How To Separate Frequently Encountered Red Stones

- Visual observation: (10x lens) Look for color, luster, cut, doublet/triplet junctions, if any.

- Determine optic character: Single refractive (SR) / Double refractive (DR) / Anomalous Double refractive (ADR) / Aggregate (AGG).

- Spectrum: Many red stones may have diagnostic spectrum.

- Microscope: Inclusions may be diagnostic, but look for inclusions that differentiate natural and synthetic, doublet / triplet.

- Dichroscope: Different cutting orientations of natural and synthetic corundum may be revealed by dichroscope.

- Fluorescence: Look under shortwave and longwave for diagnostic colors.

- Immersion cell: Use immersion cell and high refractive index liquid to separate doublets/triplets.

- Refractometer: Confirm spectroscope reading with refractometer.

The red stones, which may resemble one another in appearance and values, are:

Red Beryl

- Hardness: 7.5
- Specific gravity: 2.70 (average)
- Refractive index: 1.57 – 1.58
- Optic sign: Uniaxial negative
- Birefringence: DR; 0.006
- Other points: Inclusions, color. There are many treated stones in the gem market. Analytical techniques may be required to detect treatments.

Tourmaline

- Hardness: 7
- Specific gravity: 3.03
- Refractive index: 1.62 – 1.64
- Optic sign: Uniaxial negative
- Birefringence: DR; 0.018
- Other points: Inclusions, color, pleochroism. There are many treated stones in the gem market. Many gem quality tourmalines are relatively clean. Analytical techniques may be required to detect treatments.

Topaz

- Hardness: 8
- Specific gravity: 3.53
- Refractive index: 1.63 – 1.64
- Optic sign: Biaxial positive
- Birefringence: DR; 0.008
- Other points: Inclusions, color (pink stones may be confused for pink sapphires). There are many treated stones in the market. Most gem quality stones are relatively clean. Analytical techniques may be required to detect treatments.

Spinel

- Hardness: 8
- Specific gravity: 3.60
- Refractive index: 1.718
- Optic sign: SR
- Birefringence: -
- Other points: Inclusions, color, spectrum.

Garnet (pyrope)

- Hardness: 7.5
- Specific gravity: 3.78 (average)
- Refractive index: 1.75 (average)
- Optic sign: SR/ADR
- Birefringence: -
- Other points: Inclusions, color (may be confused for Thai ruby / synthetic ruby), spectrum.

Garnet (almandine)

- Hardness: 7.5
- Specific gravity: 4.10 (average)
- Refractive index: 1.78 (average)
- Optic sign: SR/ADR
- Birefringence: -
- Other points: Inclusions, color (may be confused for Thai ruby / synthetic ruby), spectrum.

Ruby

- Hardness: 9
- Specific gravity: 4 (average)
- Refractive index: 1.76 – 1.77 (average)
- Optic sign: Uniaxial negative
- Birefringence: DR; 0.008
- Other points: Inclusions, color, spectrum (use microscope to separate natural vs synthetic). There are many treated stones in the gem market. Analytical techniques may be required to detect treatments.

Synthetic Ruby

- Hardness: 9
- Specific gravity: 4 (average)
- Refractive index: 1.76 – 1.77 (average)
- Optic sign: Uniaxial negative
- Birefringence: DR; 0.008
- Other points: Inclusions, color, spectrum (use microscope to differentiate flux and hydrothermal). Gem quality stones are relatively clean.

Synthetic red cubic zirconia

- Hardness: 8.5
- Specific gravity: 5.65 +
- Refractive index: 2.15 +
- Optic sign: SR
- Birefringence: -
- Other points: negative refractive index, luster, color, orange flash on the pavilion, dispersion. Gem quality stones are relatively clean.

Red Zircon

- Hardness: 7.5
- Specific gravity: 4.69
- Refractive index: 1.93 – 1.99
- Optic sign: Uniaxial positive
- Birefringence: DR; 0.059
- Other points: Inclusions, color, spectrum, doubling of the back facets, dispersion, luster, negative refractive index reading. There are many treated stones in the gem market. Analytical techniques may be required to detect treatments.

Red Diamond

- Hardness: 10
- Specific gravity: 3.52
- Refractive index: 2.42
- Optic sign: SR/ADR
- Birefringence: -
- Other points: Rare, negative refractive index reading, inclusions, luster, spectrum, dispersion. There are many treated stones in the market. Most treated diamonds are relatively clean. Analytical techniques may be required to detect treatments.

Synthetic red diamond

- Hardness: 10
- Specific gravity: 3.52
- Refractive index: 2.42
- Optic sign: SR/ADR
- Birefringence: -
- Other points: Negative refractive index reading, color, inclusions, luster, spectrum, fluorescence. Synthetic diamonds are produced by the high pressure high temperature method. Gem quality stones are relatively clean. Analytical techniques may be required to identify the stones.

Glass

- Hardness: 5.5
- Specific gravity: 3.70
- Refractive index: 1.60 – 1.66
- Optic sign: SR/ADR
- Birefringence: -
- Other points: Soft, inclusions (gas bubbles), color.

Assembled Stones

Doublets / Triplets

Corundum (natural crown / synthetic base)
Other points: Immersion (Look for differences in color and luster between the sections).

Garnet topped doublet (glass)
- Refractive index: 1.76 +
- Optic sign: SR/ADR
- Birefringence: -
- Other points: Look for differences in color and luster between the sections, gas bubbles).

Synthetic spinel soude (spinel / spinel)
- Refractive index: 1.728
- Optic sign: SR/ADR
- Birefringence: -
- Other points: Look for differences in color and luster between the sections, gas bubbles).