Corundum gemstones

by Manisha G

corundum-group-of-gemstonesChemical Composition : Oxide of aluminium or “alumina”.

Crystal System : Trigonal system. The symmetry is trigonal, or threefold, along the vertical axis.

Habit: Crystals often appear to possess hexagonal symmetry. Ruby occurs commonly as a hexagonal prism, often with a striations parallel to the rhombohedron edge and with triangular striations or raised triangles on the basal pinacoid faces. Some have only the rhombohedron form which generally shows the hexagonal pyramid form with lateral striations due to alterations with the prism form. This leads to a typical barrel shaped crystal. The only other gem crystal to show lateral striations on the prism faces is quartz (trigonal system)

Varieties : Pure corundum is,colourless. The small “white sapphires” used extensively in cheaper jewellery are mostly synthetic.

Ruby : Pigeon blood-red (Burma), pinkish, purplish and brownish red, dark red.

Sapphire : Cornflower blue (Kashmir) are world famous and highly priced. Paler blue, blackish or greenish blue; Pink, yellow, green, orange, purple and other colours (some times known as fancy sapphires). It Is preferable to use the colour designation, e.g. Yellow Sapphire, Purple Sapphire etc. Orange Sapphire is called Padapradshah and is found in Sri Lanka.

Star-sapphires and star rubies are common..

Asteriated stones vary considerably in colour from a whitish grey to blue, red or black, Often the poorer the colour the better is the star, and vice versa. A fine star combined with fine colour is a rare and valuable stone. The star effect should be clear cut and the rays straight and well centred on the stone.

The Indian rubies are mostly cryptocrvstalline red corundum which are found in many parts of India. Most of these stones are not transparent.

Habit : Often hexagonal prism with triangular striations on the basal face and lateral striations on the prism faces. Some specimen show a marked parting due to repeated lamellar twinning of the cryptocrystalline layers.

Physical Properties :

Hardness : 9 on Mohs’ Scale.

Cleavage : True cleavage does not occur. However, many corundum crystals are twinned, probably due to alteration by pressure after they were formed and is called secondary twinning. “Parting” or false cleavage occasionally occurs along the junction plane between two twin lamellae. The twin planes which may be affected in this way are parallel to the basal plane.

Specific Gravity : Generally very constant at 3.99, but some sapphires may be a little higher – up to 4.01.

Refraction : Double, uniaxial with negative optic sign. R.I. -1.760 1.768 to 1.770 – 1.779. Birefringence 0.008 to 0.009.

Dispersion : 0.018

Dichroism : Variable. Strong in Burma rubies (deep red/purple-red) and such stones need to be oriented in cutting with the table parallel to the basal plane of the crystal to eliminate the undesirable yellowish tint introduced by the extraordinary ray. Purplish and brownish Siam rubies and many sapphires exhibit the property very faintly.

Colour :

The colour of corundum is allochromatic, that is to say, the colour is due to impurities and not due to an essential ingredient in the basic chemical composition. The colour of ruby is due to a small amount of Chromic oxide which replaces some of the alumina in the crystal structure. Colour in blue sapphires is due to Titanium oxide and a certain amount of Iron oxide. Green and some yellow sapphires owe their colour to the presence of small traces of chromium with some Iron oxide in variable proportions.

Absorption Spectra

The spectrum of ruby shows strong absorption of the yellow, green and violet wavelength. A series of fine lines in the 6928A0 & 6935A0, 6942A0 (one of them is a very close double line) and three clear line at 4685, 4750, 4765A0 in the blue are useful for diagnosis. The fluorescence is responsible for the deep bright red of fine ruby. It is absent in most of the Siam rubies, due to the presence of iron which surpasses the fluorescence and imparts a dull colour to the stone. Synthetic rubies have a similar spectrum to that of natural ones. Under Ultraviolet light natural and synthetic rubies show fluorescence.

Green Sapphires show a marked three band absorption in the blue, at least one of these bands is seen in most blue sapphires – a very useful diagnostic feature.

Cut :

Brilliant, step or mixed cut are common. Cabochon for star stone.

Detection :

The principal cause of confusion in this species is the vast production of synthetic in recent years. Usually these are fairly obvious but the few that are less so, may be found masquerading as real stones at any time. Methods of detection have been dealt with in detail in synthetic booklet.

Blue zoisite can easily be confused with sapphire for the colours are very similar. The refractive index (1.70) and the density (3.35) are lower and the very strong dichroism should distinguish the blue zoisite from sapphire.

Natural blue spinel may be mistaken for poor quality sapphire, as the inclusions in both are quite similar. The R.I. of 1.72 (gahno spinels of this colour may give a value up to 1.75) together with single refraction will be sufficient to distinguish them.

Synthetic blue spinel made to imitate sapphire is coloured with cobalt and will appear red under the chelsea filter and will show the distinct “cobalt” spectrum and chalky fluorescence.

Blue tourmalines and kyanite may occasionally be found in a colour approaching that of sapphire. The R.l. and other constants are very different. Extreme dichroism of tourmaline is quite remarkable.

Natural red spinel sometimes approaches the colour of ruby. The differentiation is easily made by refractometer or dichroscope. The spectroscope can be used but some red spinels show a group (not a single line) of fluorescent lines in the red. They have no lines in the blue. The synthetic red spinel shows a single fluorescent line.

Almandine-Pvrope Garnets when the refractive index is very close to that of ruby can be clearly distinguished by the typical absorption spectrum of almandine garnet and by the single refraction of the garnet.

Red tourmaline and red zircon may also be confused with ruby. However tourmaline can be detected by refractometer and the zircon will show the normal zircon spectrum.

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