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Ruby is the red variety of corundum, with a chemical composition of Al₂O₃. It often contains trace elements such as Cr, Fe, Ti, Mn, and V. Ruby belongs to the trigonal crystal system and is highly valued for its rich red color.

Historically, ruby has been regarded as the most precious gemstone in the Bible. Its fiery red hue symbolizes passion, love, beauty, loyalty, and eternity. Ruby is also the birthstone of July, representing vitality and romance.

Figure 1: Ruby

1. Refractive Index and Birefringence

The refractive index (RI) and birefringence of ruby are measured using a gem refractometer (Figure 2).

• Refractive Index (RI): 1.762 – 1.770

• Birefringence: 0.008 – 0.010

Figure 2: Gemstone refractometer

Figure 3: Refractive index readings

2. Optical Properties

Rubies are uniaxial and anisotropic. Their optical properties can be measured using a polariscope and a refractometer.

• Under a polariscope, rubies exhibit four bright and four dark positions (inhomogeneity).

• Optical sign: negative uniaxial

Figure 4: Gem polariscope

Figure 5: Uniaxial crystal interferogram

3. Pleochroism (Dichroscope)

Rubies display dichroism, meaning their color changes when viewed from different angles. Pleochroism is typically:

• Purple-red

• Orange-red

Figure 6: Dichroscope

Figure 7: Dichroism in rubies

The strength of the dichroism depends on the color intensity and depth of the ruby.

4. Absorption Spectrum

Using a gem spectroscope, rubies show characteristic absorption features due to trace elements:

• Red region: 694 nm, 692 nm, 668 nm, 659 nm

• Orange-yellow region: 620–540 nm

• Blue region: 476 nm, 468 nm

• Violet region: full absorption

Figure 8: Prism and grating spectroscopes

Figure 9: Ruby absorption spectrum

5. Density

Ruby’s density can be measured with a density balance:

• Typical density: 4.00 g/cm³

• Density varies with trace elements like Cr and Fe

Figure 10: Density balance

6. Magnified Inspection (Microscope)

Using a gemological microscope, ruby can be analyzed for inclusions and treatment evidence (Figure 11).

A. Natural Ruby Inclusions

• Gas-liquid inclusions

• Fingerprint-shaped inclusions

• Mineral inclusions

• Color zoning or bands

• Growth lines and twin lines

• Negative crystals

• Filamentary and needle-like inclusions

• Foggy inclusions

B. Treated (Filled) Rubies

• Surface gloss differs from main gem

• Bubbles and flashes at filled areas

• Detected by IR/Raman spectroscopy

• UV fluorescence shows filling distribution

• XRF can detect abnormal foreign elements (lead, hydride, etc.)

C. Dyed Rubies

• Uneven color distribution

• Color concentrated in cracks or surface depressions

• Special fluorescence under long- and short-wave UV light

• Pleochroism often weak or absent

• UV-Vis spectrum abnormalities

• Color removable by solvents (acetone, ethanol)

D. Synthetic Rubies

1. Flame-Fusion Ruby

   • Bubbles

   • Arc-shaped growth lines

2. Flux-Process Ruby

   • Flux inclusions

   • Platinum flakes (triangular/hexagonal)

   • Comet-shaped inclusions

   • Syrup-like texture

3. Hydrothermal Ruby

   • Dendritic growth lines

   • Color bands

   • Golden metal flakes

   • Transparent gauze-like inclusions or nail-shaped inclusions

Figure 11: Ruby under gemological microscope

✅ Key Takeaways

• Use refractometer, polariscope, dichroscope, spectroscope, and microscope to identify ruby.

• Natural rubies have characteristic inclusions and pleochroism.

• Treated rubies often show oil, resin, or filling traces.

• Synthetic rubies have distinct growth patterns and inclusions depending on the manufacturing process.

• Dyed rubies can be detected by uneven color, fluorescence, and chemical testing.

With these techniques, you can confidently distinguish natural, treated, synthetic, and imitation rubies in the market.