Spinel gemstone

by Manisha G

image of Spinel gemstoneChemical composition: Magnesium aluminium oxide.

Crystal system: Cubic system.

Habit: Usually as octahedron, rarely in dodecahedron or cube forms. Spinel twins (macles or twinned octahedron) are common. Octahedral faces have often a bright natural lustre but may show triangular growth markings.

Varieties:

Red Spinel – of a color approaching that of a ruby is the most prized as a gem. But names like “spinel-ruby” or “balas-ruby” should not be used. Other reds are more common and may tend to orange, brownish or purplish shades. These usually do not show the chrome-spectrum seen in the red stones, or show it very weakly.

Blue Spinel – varies from a fine blue to pale greyish blue to violet and through a great many shades to deep blue black. Greenish colours are also known.

Star stones – these are also known but are much rarer than in corundums.

Ghanospinels – are blue or mauve spinel containing appreciable amount of zinc. These have higher S.G. and R.I. (S.G.-3.58 to 4.40 and R.I. – 1.725 to 1.835). A color changing natural spinel with a faint colour change from pinkish blue to pale purple has been reported but this gem cannot be confused with Alexandrite or even with natural colour changing sapphire.

Physical properties:

Cleavage: Imperfect

Hardness: 8

S.G.: 3.58 to 3.64 mean (3.60)

Lurtre: Vitreous

Refraction: Single

R.I.: 1.712 to 1.80 (mean 1.718)

Both S.G. and R.I. vary within a certain range, but are very constant for red stones.

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