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Introduction of Ruby

Figure 1 Ruby

The mineral name of ruby is corundum, and the red corundum gemstone is ruby. Its chemical composition is aluminum oxide (Al2O3), containing trace elements such as Fe, Ti, Cr, Mn, and V, and belongs to the trigonal crystal system.

In the Bible, ruby is the most precious of all gemstones. The fiery red color of ruby makes people always associate it with passion and love. It is known as the “stone of love”, symbolizing the passion, beauty, eternity and loyalty of love. Ruby is the birthstone of July.

Detailed Identification

Only when a gemstone that meets all the following identification characteristics is a ruby.

Refractive index and birefringence of ruby

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

The refractive index (Figure 3) is: 1.762~1.770, and the birefringence is: 0.008~0.010.

Figure 2 Gemstone refractometer	Figure 3 Refractive index readings

Optical characteristics of ruby

The optical characteristics of ruby are mainly detected by a gem polariscope (Figure 4).

The optical characteristics of ruby are: inhomogeneity (four bright and four dark under a polariscope), uniaxial crystal (Figure 5), and negative optical properties (measured by a refractometer).

Figure 4 Gem polariscope	Figure 5 uniaxial crystal interferogram

Pleochroism of Ruby

Ruby has dichroism, which is measured by a dichroscope (Figure 6). The strength of ruby’s dichroism and the color change depend on the color and color depth of the ruby itself. Common ruby dichroism (Figure 7) is: strong, purple-red, orange-red.

Figure 6 Dichroscope	Figure 7 Dichroism of rubies

Ruby Absorption Spectrum

The absorption spectrum of ruby is measured by a spectroscope (Figure 8). Different rubies have different absorption spectrum depending on the impurities contained in the ruby. Generally speaking, ruby has absorption lines at 694nm, 692nm, 668nm, and 659nm, an absorption band at 620-540nm, absorption lines at 476nm and 468nm, and full absorption in the purple region (Figure 9).

Figure 8 Prism spectroscope and grating spectroscopes	Figure 9  Absorption spectrum of rubies

Ruby Density

The density of ruby is measured by a density balance (Figure 10). The density of ruby is 4.00g/cm3. The content of trace elements such as Cr and Fe affects the density value. The higher the content, the greater the density.

Figure 10 density balance

Magnify for inspection

The magnified features of ruby can be measured using a gemological microscope (Figure 11).

Figure 11 Gemological microscope

1) Characteristics of natural ruby: gas-liquid inclusions, fingerprint-shaped inclusions, mineral inclusions, color bands, growth lines, twin lines, negative crystals, filamentary inclusions, needle-shaped inclusions, and foggy inclusions.

2) Characteristics of filled rubies: Magnified inspection shows that the surface gloss of the filled part is different from that of the main gemstone, and flashes and bubbles can be seen at the filled part; infrared and Raman spectroscopy tests show the characteristic peaks of the filling; luminescence image analysis (such as ultraviolet fluorescence observation) can observe the distribution of the filling; composition analysis instruments (such as XRF, etc.) can detect abnormal content of foreign elements (such as lead, hydride, etc.).

3) Characteristics of dyed ruby: Magnified inspection shows that the color distribution is uneven, and is mostly concentrated between cracks or in depressions on the surface; the dye can cause special fluorescence under long- and short-wave ultraviolet light; there is no obvious pleochroism; abnormalities are visible in the ultraviolet-visible spectrum; the color can be removed by wiping with solvents such as acetone or anhydrous ethanol.

4) Characteristics of synthetic ruby:

Flame-fusion ruby: bubbles, arc-shaped growth lines.

Flux-process ruby: flux inclusions, platinum metal flakes in triangles and hexagons, comet-shaped inclusions, and syrup-like texture.

Hydrothermal ruby: dendritic growth lines, color bands, golden metal flakes, colorless and transparent gauze-like inclusions or nail-shaped inclusions.