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Spinel is a gemstone with a long history. In ancient times, it was always mistaken for ruby. The most legendary and fascinating spinels in the world are the 361ct “Timur Ruby” and the 170ct “Black Prince’s Ruby” inlaid in the crown of the King of the British Empire in 1660. It was not until modern times that they were identified as red spinels. The “rubies” used on the hats of the first-rank officials in the Qing Dynasty in China were almost all made of red spinel.

Spinel belongs to the spinel family in mineralogy. It may contain trace elements such as Al, Cr, Fe, Zn, and Mn. It belongs to the isometric system and is often octahedral. Sometimes octahedrons are aggregated with rhombic dodecahedrons and cubes to combinate form. Spinel can be red, orange-red, pink, purple-red, colorless, yellow, orange-yellow, brown, blue, green, purple, and many other colors (Figure 1).

Figure 1 Spinel in various colors

Identification methods

Only gemstones that meet all the following identification characteristics datas of spinel are spinel (please visit www.szfable.com to learn about gem identification instruments).

1. Refractive index of spinel

The refractive index of spinel (Figure 3) is: 1.718 (+0.017, -0.008). The refractive index of zinc spinel is 1.805, iron spinel is 1.835, and chrome spinel can be as high as 2.00. There is no double refraction.

Figure 2 Gemstone refractometer

Figure 3 Refractive index readings

Optical characteristics of spinel

The optical characteristics of spinel are mainly detected through a gem polariscope (Figure 4). Spinel is a isotropic body and is completely dark or anomalous extinction when rotated 360° under an crossed polariscope (Figure 5).

Figure 4  gem polariscope

Figure 5 full dark

3.Spinel has no pleochroism.

4.Density of spinel

The density of the spinel was measured by the density balance (Figure 6). The density of spinel is 3.60(+0.10,-0.03)g/cm3.

Figure 6 density balance

5. Luminescence of spinel

The luminescence of spinel is measured by ultraviolet fluorescent lamp (Figure 7). Red, orange, and pink spinel have weak to strong red and orange fluorescence under long-wave ultraviolet light; no to weak red and orange fluorescence under short-wave ultraviolet light. Yellow spinel has weak to moderate brownish yellow fluorescence under long-wave ultraviolet light; no to brownish yellow fluorescence under short-wave ultraviolet light. Green spinel has no to moderate orange-orange red fluorescence under long-wave ultraviolet light. Colorless spinel has no fluorescence.

Figure 7 ultraviolet fluorescent lamp

Figure 8 Ultraviolet fluorescence of spinel

6. Spinel absorption spectrum

The absorption spectrum of spinel is measured by a spectroscope (Figure 9). Spinels of different colors have different absorption spectra. Red and pink spinels are colored by the Cr element, and their absorption spectra have strong absorption bands of 595-490nm in the yellow-green region; strong absorption lines of 685nm and 684nm and weak absorption bands of 656nm in the red region. In the fluorescence spectrum, the absorption lines of red spinel in the red region are bright fluorescent lines, which are different from a group of thin lines in ruby. Spinel has more than 10 bright fluorescence lines, with the strongest absorption lines at 686nm and 675nm. The color-causing elements of blue and purple spinel are Pe or a small amount of Co. Their main absorption lines are in the blue area, 460nm with strong absorption band, 430～435nm, 480nm, 550nm, 565—575nm, 590nm, 625nm with weak or very weak absorption line or band. The 460nm absorption band is not found in synthetic blue spinel. The absorption spectrum of zinc spinel is similar to that of blue spinel, but weaker.

Figure 9  Prism spectroscope and grating spectroscope

7. Internal and external microscopic features of spinel

Magnified features of spinel measured using a gemological microscope (figure 10).

figure 10 gemological microscope

1) Solid inclusions

Common octahedral spinel inclusions are single, arranged in rows or distributed in the shape of fingerprints. Sometimes octahedral negative crystals are seen, which are partially filled with calcite and dolomite. Other inclusions include flake graphite, columnar apatite, quartz, etc. In spinels produced in Myanmar, small haze-like inclusions and blade-shaped sphene inclusions are found, which can form a starlight effect when dense.

Figure 11  Crystal inclusions

 2) Liquid inclusions

Liquid inclusions are common in open cracks. Fingerprint-shaped inclusions formed by tension cracks may be found around octahedral crystal inclusions. The zircon crystal inclusions contained in Sri Lankan spinel have brown spots on the periphery, which were once thought to be irradiation spots of zircon. The new view is that these are tension cracks caused by uneven thermal expansion between the intermediate mineral crystals and the main crystals.

3) Growth phenomenon

Growth bands and twinning patterns along the octahedral crystal planes can be seen, which are most easily observed by oil immersion observation under crossed polorizer.