- 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 blue 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 blue stones, which may resemble one another in appearance and values, are:
Synthetic blue quartz
- Hardness: 7
- Specific gravity: 2.65
- Refractive index: 1.54 – 1.55
- Optic sign: Uniaxial positive
- Birefringence: DR; 0.009
- Other points: Color, inclusions, interference figures, if seen, spectrum. Common look-alikes include aquamarine, blue spinel, blue sapphire, blue topaz, and glass. Standard / analytical techniques may be required to identify the stones.
Iolite
- Hardness: 7.5
- Specific gravity: 2.60
- Refractive index: 1.54 – 1.55
- Optic sign: Biaxial negative
- Birefringence: DR; 0.009
- Other points: Color, pleochroism, inclusions. Iolite may be treated. Common look-alikes include amethyst, tanzanite, blue sapphire, blue spinel, blue tourmaline, tanzanite and glass. Standard / analytical techniques may be required to identify the stones.
Beryl (aquamarine)
- Hardness: 7.75
- Specific gravity: 2.70-2.90
- Refractive index: 1.56 – 1.59
- Optic sign: Uniaxial negative
- Birefringence: DR; 0.005/9
- Other points: Color, inclusions. Aquamarine may be treated. Common look-alikes include blue topaz and glass. Standard / analytical techniques may be required to identify the stones.
Beryl (maxixe-type)
- Hardness: 8
- Specific gravity: 2.70 – 2.90
- Refractive index: 1.56 – 1.59
- Optic sign: Uniaxial negative
- Birefringence: DR; 0.005/9
- Other points: Color (unstable color; fades), inclusions. Maxixe type beryl may be treated, and may look like aquamarine, tanzanite, synthetic blue quartz, and glass. Standard / analytical techniques may be required to detect treatments / identify the stones.
Blue topaz
- Hardness: 8
- Specific gravity: 3.56
- Refractive index: 1.61 – 1.62
- Optic sign: Biaxial positive
- Birefringence: DR; 0.010
- Other points: Color. Blue topaz may be treated. Most gem quality stones are relatively clean. Blue topaz may look like aquamarine, blue sapphire, blue spinel, iolite, synthetic blue quartz and glass. Standard / analytical techniques may be required to detect treatments / identify the stones.
Apatite
- Hardness: 5
- Specific gravity: 3.18
- Refractive index: 1.63 – 1.64
- Optic sign: Uniaxial negative
- Birefringence: DR; 0.002- 4 (average)
- Other points: Color, spectrum, dichroism, inclusions. Gem quality stones may be relatively clean. Apatite may look like blue spinel, blue sapphire, blue topaz, blue tourmaline, and glass. Standard / analytical techniques may be required to identify the stones.
Tourmaline
- Hardness: 7
- Specific gravity: 3.05
- Refractive index: 1.62 – 1.64
- Optic sign: Uniaxial negative
- Birefringence: DR; 0.018
- Other points: Color, inclusions, pleochroism, doubling of back facets. Tourmaline may be treated. Gem quality stones are relatively clean. Common look-alikes include blue sapphire, blue spinel, apatite, blue topaz, tanzanite, iolite and glass. Standard / analytical techniques may be required to detect treatments / identify the stones.
Tanzanite (Zoisite)
- Hardness: 6.5
- Specific gravity: 3.35
- Refractive index: 1.69 – 1.70
- Optic sign: Biaxial positive
- Birefringence: DR; 0.009
- Other points: Color, pleochroism, inclusions. Tanzanites may be treated. Gem quality stones relatively clean. Common look-alikes include blue sapphire, iolite, tourmaline, amethyst and glass. Standard / analytical techniques may be required to detect treatments / identify the stones.
Spinel
- Hardness: 8
- Specific gravity: 3.60+
- Refractive index: 1.718
- Optic sign: SR
- Birefringence: -
- Other points: Color, spectrum, inclusions. Gem quality blue spinels may be clean. Common look-alikes include blue sapphire, blue tourmaline, iolite, tanzanite, amethyst and glass. Standard / analytical techniques may be required to identify the stones.
Synthetic blue spinel
- Hardness: 8
- Specific gravity: 3.63
- Refractive index: 1.728
- Optic sign: SR
- Birefringence: -
- Other points: Color, inclusions, Chelsea color filter, fluorescence. Standard identification techniques may be required to identify the stones.
Sapphire
- Hardness: 9
- Specific gravity: 4
- Refractive index: 1.76 – 1.77
- Optic sign: Uniaxial negative
- Birefringence: DR; 0.008
- Other points: Color, inclusions, fluorescence, spectrum. Use microscope to separate natural from synthetic. Sapphires may be treated. Gem quality stones are relatively clean. Common look- alikes include tanzanite, blue spinel, blue topaz, blue tourmaline, synthetic blue quartz, iolite and glass. Standard / analytical techniques may be required to detect treatments / identify the stones.
Synthetic sapphire
- Hardness: 9
- Specific gravity: 4
- Refractive index: 1.76 – 1.77
- Optic sign: Uniaxial negative
- Birefringence: DR; 0.008
- Other points: Color, inclusions, fluorescence, spectrum. Use microscope to separate flame fusion from flux from hydrothermal stones. Gem quality stones are relatively clean. Analytical techniques may be required to identify the stones.
Benitoite
- Hardness: 6.5
- Specific gravity: 3.67
- Refractive index: 1.76 – 1.80
- Optic sign: Uniaxial positive
- Birefringence: DR; 0.047
- Other points: Color, inclusions, fluorescence, spectrum, dispersion. Standard / analytical techniques may be required to identify the stones.
Blue zircon
- Hardness: 7.5
- Specific gravity: 4.69
- Refractive index: 1.93 – 1.99
- Optic sign: Uniaxial positive
- Birefringence: DR; 0.059
- Other points: Color, inclusions, spectrum, doubling of back facets, negative refractive index reading, luster, abraded facet junctions. Common look-alikes include blue sapphire, blue topaz, aquamarine, synthetic blue quartz and glass. Gem quality stones are relatively clean. Standard / analytical techniques may be required to detect treatments / identify the stones.
Blue diamond
- Hardness: 10
- Specific gravity: 3.52
- Refractive index: 2.42
- Optic sign: SR/ADR
- Birefringence: -
- Other points: Rare; Color, inclusions, fluorescence, spectrum. Blue diamonds may be treated. Gem quality stones are relatively clean. Analytical techniques may be required to detect treatments.
Synthetic blue diamond
- Hardness: 10
- Specific gravity: 3.52
- Refractive index: 2.42
- Optic sign: SR/ADR
- Birefringence: -
- Other points: Color, inclusions, fluorescence, spectrum. Synthetic blue diamonds are produced by high pressure high temperature method. Gem quality stones are relatively clean. Analytical techniques may be required to identify the stones.
Synthetic blue cubic zirconia
- Hardness: 8.5
- Specific gravity: 5.65 +
- Refractive index: 2.15 +
- Optic sign: SR
- Birefringence: -
- Other points: Color, negative refractive index reading, luster, dispersion. Standard / 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
- Birefringence: -
- Other points: Color, soft, inclusions (gas bubbles, swirls), luster, Chelsea color filter (for cobalt rich stones), spectrum. Standard / analytical techniques may be required to identify the stones.
Assembled Stones
Doublets / Triplets
Corundum composites (natural crown/synthetic base)
Refractive index: 1.76 – 1.77
Birefringence: DR; 0.008
Other points: Immersion (Look for differences in color and luster between the sections)
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)
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)
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