Information on Topaz gemstone

by Manisha G

Topaz:

types-of-topaz-gemstones

Chemical composition: Fluosilicate of aluminium.

Fluosilicate of aluminium in which fluorine is in part replaced by the hydroxyl molecule (OH). This is one of the few gem minerals that contains fluorine. The yellow, brown and pink stones are generally the hydroxyl-rich topazes, while the white and blue varieties contain more fluorine (F).

Crystal system: Orthorhombic.

Habit: Prismatic habit, usually striated along the length of the prism faces; but sometimes all faces are smooth, with a light “frosted” surface. Colourless topaz often show compplex terminations of pyramids, domes and basal pinacoid. One end of the crystal is usually flat because topaz normally occurs firmly attached to the parent rock, and when broken, shows the characteristic rhombic shape of the prism cross-section due to perfect basal cleavage.

Varieties: Colourless, pink and shades of yellow to sherry-brown are the varieties normally encountered in trade. Blue and green-blue may resemble aquamarine. Red and pink are extremely rare as natural colours. Pink topaz is the result of the artificial process called “pinking” i.e. suitable sherry coloured crystals are heat-treated before cutting.

 

Physical properties:

Cleavage: Highly perfect, parallel to the basal plane. Hence the stone has a tendency to develop flaws if carelessly handeled. Such flaws are cracks, often showing brilliant irridescent colours and lessening considerably the beauty and value of the stone. Cutting must be carefully carried out as the stone is liable to chip at the edges and to flaw with any jarring of the cutting lap. A topaz crystal can be split with comparative case in the direction parallel to the basal plane and long drop-shaped faceted stones readily break across if carelessly handled.

Hardness: 8 on Moh’s scale.

Specific gravity: 3.53 – 3.56. The yellow, brown and pink varieties used in jewellery have S.G. near 3.53 while colourless and pale blue topaz contains more fluorine, giving the higher S.G. upto 3.56. Similarly some light brown stones may also be fluorine rich and have the higher S.G. Diamond has similar S.G.

Lusture: Vitreous, but particularly bright.

Refraction: Double, biaxial positive sign. Yellow, brown and pink topaz; R.I.:1.630 – 1.638. White and blue topaz; R.I.: 1.61 – 1.62 (D.R.: 0.008 – 0.010)

Note: The presence of fluorine raises the S.G. and birefringence, but lowers the refractive indices.

Dichroism: Generally distinct in dark coloured stones. Differences may be difficult to detect in pale specimens.

Spectrum: Pink topaz may show faint chromium absorption lines in the red due to chromium (6820A).

Other important points:

In the market, much confusion exists over this stone, and stones of other species are often wrongly called topaz. Topaz can be readily distinguished from citrine quartz (“golden topaz” of the market) by the fact that the later floats on methylene iodide while the former sinks. This also distinguishes topaz from yellow beryl or yellow tourmaline. Topaz has a far greater heft than any of the above mentioned gems. Yellow and fancy orange sapphires may be confused with topaz but their higher S.G. and R.I. easily distinguish them.

Colour in Brazilian yellow and brown stones is due, atleast in part, to chromic oxide. Some yellows from other localities are probably due to other oxides or to defects in the lattice and are said to fade on long exposure to light or heat.

Cut: Brilliant, step and mixed cut. Arge stones are often modified by additional facets.

Treatments: Heat, irradiation, oiling.

Detection: Almost any yellow or sherry-brown gem may at some time masquerade as topaz. These may include citrine quartz, beryl, tourmaline, chrysoberyl, sapphire, hessonite garnet, zircon and among the rare gems, such stones are orthoclase feldspar, apatite, danburite or even sphene. Most of these are not really like topaz beyond the fact of their yellow colour, and a refractometer will distinguish all of them without difficulty with the possible exception of yellow tourmaline and danburite. Yellow tourmaline, however, has twice the birefringence of topaz, a fact which is easily seen on the refractometer. The case of danburite is different; for the double refraction is only a little less than for topaz (0.006 for danburite and 0.008 for topaz) so greater care needs to be taken, but if unset the lower densities of these stones will provide an answer.

Citrine resembles topaz more closely than other yellow or brown gems. This stone is identified quite simply by its much lower R.I., S.G. and bull’s eye effect, but occasion may arise when a test cannot conveniently be made and preliminary opinion must be arrived at by visual examination only. Careful observation will show that the luster (or polish) of topaz is brighter than that of the quartz gem. The colour of citrine is often zoned or patchy while that of topaz is more likely to be evenly distributed. The heft of topaz (S.G.: 3.53) will be perceptibly felt when held in the hand than a citrine (2.65) of similar size.

Blue and pink topaz are likely to be confused with the blue and pink tourmaline. Again S.G. and R.I. are the best test to identify them. The greater birefringence of the tourmaline is easily seen on the refractometer and, in large specimens, may be noticeable in the form of doubling of the back facets. (In this connection it should be borne in mind that any birefringent stone, if large enough, will quite possibly show this doubling to some extent, even in a stone like topaz with only 0.008 of birefringence. Sometimes bigger size citrine may be confused as zircon, on the strength of slight “doubling” seen with a lens).

Synthetic spinels with colours very similar to topaz (blue, sherry-brown and pink) have been seen in the market. The R.I. (1.727), S.G. (3.72 appx.), UV shows chalky fluorescence for synthetic blue spinel, lack of pleochroism and the strain seen under the polariscope as well as the characteristic spectrum will identify them quietly easily.

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