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Spinel is a gemstone with a fascinating and often misunderstood history. For centuries, many of the world’s most famous “rubies” were later discovered to be red spinels.

Among the most legendary examples are the 361-carat Timur Ruby and the 170-carat Black Prince’s Ruby, both set in the British Crown Jewels since 1660. Modern gemology eventually revealed that these historic stones are actually red spinels.
Similarly, many of the “rubies” worn on the ceremonial hats of high-ranking officials during China’s Qing Dynasty were also red spinels.

Today, spinel is finally recognized and appreciated in its own right — not only for its beauty, but also for its remarkable gemological properties.

1. What Is Spinel?

Spinel belongs to the spinel mineral group and crystallizes in the isometric (cubic) crystal system. It commonly forms octahedral crystals, sometimes combined with rhombic dodecahedrons or cubes.

Chemically, spinel may contain trace elements such as:

• Aluminum (Al)

• Chromium (Cr)

• Iron (Fe)

• Zinc (Zn)

• Manganese (Mn)

These trace elements are responsible for spinel’s wide color range.

Common Spinel Colors

Spinel occurs in many colors, including:

• Red

• Orange-red

• Pink

• Purple-red

• Colorless

• Yellow

• Orange-yellow

• Brown

• Blue

• Green

• Purple

Figure 1 shows spinel in various natural colors.

2. Key Identification Criteria for Spinel

Only gemstones that meet all of the following gemological characteristics can be identified as natural spinel.

3. Refractive Index (RI)

Spinel has no double refraction (isotropic).

• Standard spinel RI: 1.718
  (+0.017 / −0.008)

Special varieties:

• Zinc spinel: ~1.805

• Iron spinel: ~1.835

• Chromium spinel: up to ~2.00

Measurement is performed using a gemstone refractometer .

4. Optical Properties (Polariscope Test)

Spinel is an isotropic gemstone.

When examined under a crossed polariscope:

• The stone remains completely dark or shows anomalous extinction

• This behavior is consistent through a full 360° rotation

This feature helps distinguish spinel from ruby and other doubly refractive gemstones.

5. Pleochroism

Spinel shows no pleochroism.

When viewed with a dichroscope:

• No color change is observed

• This is an important difference from ruby and sapphire

6. Density (Specific Gravity)

Spinel has a relatively high and stable density.

• Density: 3.60 g/cm³
  (+0.10 / −0.03)

Measured using a hydrostatic balance or density balance .

7. Ultraviolet Fluorescence

Spinel fluorescence varies by color and composition.

Under Long-Wave UV:

• Red, pink, orange spinel: weak to strong red/orange fluorescence

• Yellow spinel: weak to moderate brownish-yellow fluorescence

• Green spinel: none to moderate orange-red fluorescence

• Colorless spinel: no fluorescence

Under Short-Wave UV:

• Generally none to weak fluorescence across most colors

(demonstrate UV testing and fluorescence results.)

8. Absorption Spectrum of Spinel

The absorption spectrum is examined using a prism or grating spectroscope .

Red & Pink Spinel (Cr-colored):

• Strong absorption bands at 595–490 nm (yellow-green region)

• Strong absorption lines at 685 nm and 684 nm

• Weak absorption at 656 nm

Under fluorescence observation:

• Red spinel shows bright fluorescent lines, unlike ruby

• Spinel typically displays more than 10 bright fluorescence lines

• Strongest lines at 686 nm and 675 nm

Blue & Purple Spinel (Fe or Co):

• Strong absorption band at 460 nm

• Additional weak bands at:

  • 430–435 nm

  • 480 nm

  • 550 nm

  • 565–575 nm

  • 590 nm

  • 625 nm

⚠️ The 460 nm absorption band is absent in synthetic blue spinel, making this a valuable identification feature.

Zinc Spinel:

• Absorption spectrum similar to blue spinel but noticeably weaker

9. Microscopic Features of Spinel

Microscopic examination is performed using a gemological microscope .

9.1 Solid Inclusions

Common inclusions include:

• Octahedral spinel crystals (single, aligned, or fingerprint-like)

• Octahedral negative crystals partially filled with calcite or dolomite

• Flake graphite

• Columnar apatite

• Quartz inclusions

Myanmar spinel may show:

• Haze-like inclusions

• Blade-shaped sphene inclusions

• Dense inclusions capable of producing a star (asterism) effect

(Figure 11 shows typical crystal inclusions.)

9.2 Liquid Inclusions

• Frequently found along open fractures

• Fingerprint-like patterns may form around crystal inclusions

• Sri Lankan spinel often contains zircon inclusions surrounded by brown tension halos

These halos are now believed to be caused by uneven thermal expansion, not radiation damage.

9.3 Growth Features

Natural spinel may display:

• Growth zoning

• Twinning along octahedral planes

These features are most clearly observed using oil immersion under crossed polarizers.

Final Thoughts

Spinel is not merely a “ruby substitute” — it is a historically important and scientifically fascinating gemstone with its own identity. By understanding its optical behavior, physical properties, spectral features, and inclusions, you can confidently distinguish spinel from ruby, synthetic stones, and imitations.

Whether for gem trading, collection, education, or jewelry appreciation, mastering spinel identification ensures accuracy, value protection, and deeper appreciation of this remarkable gemstone.