Spinel Identification

by Erum Qureshi

A gemstone purported to be a spinel undergoes stringent tests in the lab for authenticity and grades (i.e. quality). It has distinctive characteristics and traits that a gemologist can easily identify.

image courtesy: Orbital Joe
The mineral spinel produces a lovely suite of gemstones which are overshadowed by the more opulent ruby and sapphire. After ruby, spinel is perhaps the most beautiful of all red gems. Its colors vary from the lightest pink to a deep ‘garnet’ red. A beautiful ‘flame’ orange, magnificent cobalt-blue and a subdued blue-green are also some color varieties of this mineral.
The derivation of the name’ spinel’ is obscure; its origins lie probably in the Latin ‘spina’, meaning thorn. It may also be derived from the Greek word, meaning spark, so christened because of its fiery red color.

Species

Spinel is a gem species by itself and is an aluminate of magnesium. Other members of the spinel group are Gahnospinel, (dark green), Gahnite (blue, dark blue), Hercynite (dark to black), Ceylonite or pleonaste (dark colors), Picotite (dark green to black) and Galaxite (dark red to black in color).

Hardness

Spinel is comparatively hard, (rating 8 on the Mohs scale of hardness). It shows an uneven cleavage and a conchoidal fracture and has a brittle nature.

Determination of Specific Gravity

Each gemstone has its own specific gravity or ratio of its weight to the same volume of water.

The specific gravity is measured by immersing the gem (spinel in this case) in a series of heavy liquids, (usually a set of three liquids of varying densities). The gem will float in a liquid of higher density, sink in a liquid of lower density and remain suspended in a liquid of the same density. Although the density/specific gravity of spinel varies from 3.58 to 3.98, gem grade spinel range from 3.58 to 3.61. Pale pink stones have much lower values.

Refractive Index

Light rays leaving one medium and entering another obliquely seem to bend a little at the place of contact. This is called refraction of light. Refractive indices for spinel are measured by placing the stone face down in a Refractometer and a reading is taken. The mineral spinel has an RI in the range of 1.712 to 1.80. However gem quality spinel will have a fairly consistent RI, steady at 1.718. In red stones rich in chromium, it may rise up to 1.74. Ceylonites have values from 1.77 to 1.80.

Absorption Spectra

The absorption spectrum of red and pink spinel is typical of stones, which owe their color to chromium. In a spectroscope, which is an instrument used to determine the wavelength of the absorbed white light when it is passed through the gemstone, the spectrum is characterized by a broad absorption in the yellow-green, and absorption in the violet. There are no lines in the blue window and faint ones in the red. The wavelength is measured in nanometers (symbol nm) or angstrom symbol ?. The absorption spectra of red spinel (in nanometers): 685, 684, 675, 665, 656, 650, 642, 632, 595-490, 465, 455. (Strong absorption lines are in bold letters; weak ones are in parenthesis.)

Fluorescence

The luminescence of spinel shows considerable variation. Natural red and pink spinel show a strong red glow under long-wave UV light, and weaker under short-wave fluorescence. Dark blue spinel are completely inert under any radiation.

Microscopic Examination

As with any other gemstone, the final call on its identification and indeed its authenticity depends heavily on its microscopic examination and the ability of the gemologist to successfully differentiate between natural inclusions and those commonly associated with synthetics. Common natural spinel inclusions include ‘spangles’ which are included crystals with surrounding iridescent stress fractures. Other common inclusions are octahedral crystals or cavities filled with calcite or dolomite. Synthetic spinel have inclusions which include gas bubbles with torpedo shapes and long parallel hose-like tubes. Most (but not all) synthetic red spinel display obvious curved growth lines which appear as broad swathes of color.