ASTERISM
Asterism” is a term, which is most commonly used in gemmology. It refers to a kind of special optic phenomenon that has been seen in several gems.
When parallel, needle-like inclusions, or tube-like channels, are oriented along two or more of the crystal faces of a mineral, and when that stone is cut as a domed cabochon, a four to six rayed figure is displayed on the dome. This phenomenon of reflected light is called “asterism” and the gems are called star stones. The most commonly seen examples are star corundum; where there are inclusions of titanium oxide (rutile or “silk”) parallel to three crystal faces giving a six rayed star. In rare cases a twinned crystal slightly offset with its own. set of rutile needles can lead to the formation of a twelve rayed star.
Although rutile is an extremely common inclusion in sapphire. few good, natural, star sapphires are found. One of the major reasons is that the heating which is almost universally done to sapphire rough, dissolve rutile needles, clarifying and sometimes color enhancing the stone, yes, but eliminating potential stars Special optic phenomena refer uncommon and excellent optic properties exhibit by some rare gemstone. The phenomena are particular property of transmitting or reflecting light. The structure or inclusions of a mineral in gems is the reason of this property. The beauty and value of gems depend on their optical properties.
“Asterism” in one of the important optical phenomena has been seen in sapphires, rubies, rose quartz and some other gems. The term “Asterism” came from the Greek word “aster”, which means star. From the name we can guess “asterism” is star-shaped light effect across the surface of gems.
SPECIAL OPTICAL PHENOMENON
“Asterism” is an exclusive light effect (which we called. special optical phenomenon) of some crystal. When light pass through these crystals, its exhibit a star-shaped figure, which called “asterism”. This figure is produced by reflected or transmitted light.
ASTERISM IN GEMS
Gems, which show an illuminated star, just below the surface of crystal called star gems. The most popular star gems are star ruby and star sapphire. Other gems, which also possess the special optic phenomenon “asterism”, are rose quartz, garnet, spinel, Chrysoberyl and Diopside. There are a few specimen of rock crystal, which has the phenomenon.
The star in the star gems may have four to twelve rays depending on the crystal system of the gems. But the six- rayed star effect is most common. Usually, six-rayed star show by the star rubies and star sapphires. Sometimes these star gems exhibit the twelve-rayed star effect. But the twelve-rayed star effect in corundum is occasional. Rose quartz exhibits six-rayed star. Some garnets and spinel commonly have the star effect with four rays, but these star gems can show six-rayed star effect if cut in a distinct way. Some crystals like Diopside only exhibit four-rayed star effect.
“Asterism” is caused by the dense inclusions of tiny fibres of foreign minerals in gems. Generally, these inclusions are the reason of the reflection of light and form a wavy star like formation, which moves around when the mineral is rotated. All the inclusions in the gems are formed via exsolution.
The inclusions must be long, very thin needle like which called as rutile. “Asterism” is produced when needles intersect each other in multiple groups, Small needles of rutile (titanium oxide), sometimes called “silk” are oriented inside the minerals in the direction of the crystal growth system. There is a relationship between the gem’s growth and the number of rays produced by the reflected light.
There must be three or more chatoyant bands inside the gems to produce “asterism”. The arrangement of needles in a hexagonal pattern produced six-rayed “asterism”.
Exsolved inclusions form a pattern in the host mineral. The pattern may vary from mineral to mineral, but in case of specific mineral the pattern remain same. In case of corundum, we can see rutile is exsolved in three directions crossing at 60 degree or 120 degree in the basal and produce six or twelve-rayed star. When the rutiles crossed at 90 degree, it produces a four-rayed star. Star gems are translucent to opaque. The rays of star should cross at the top of these gems and be distinct.
The sharpness and the size of “asterism” in gems, which identified as phenomenon, depend on the quality and quantity of rutile needles. Small to smaller size of rutile needles help “asterism” to be sharper. When the needles are visible with open eyes as inclusions, the gems turned into a low translucent stone and lack “asterism”.
Large quantities of rutile needles in a crystal produce phenomenal “aster”. If the quantities of rutiles are small, “Asterism” is visible in only a small area. These gems are not recognized as phenomenal gems .The clarity of these gems can be improve by dissolving some rutile needles with heat treatment. Heat treatment allows various elements to be incorporated into crystal. Then rutile needles form by very gradual cooling system and induce “asterism”.
Inclusions are not present in all star gems. Some specimens leave holes or empty areas in the host. Reflection creates from fibres or fibrous cavity in the crystal.
TYPES OF ASTERISM
There are two types of “asterism”, like Epiasterism and Diasterism. Epiasterism is creates by the reflection of light on parallel-arranged inclusions inside the crystal. Diasterism is creates by the transmitting light through the stone, mostly seen in rose quartz. To see this effect a lighted background is essential.
To display the star effect in best way a gem is cut as cabochon with the girdle parallel to the needles. The domed stones are round or oval shaped.
The special optical phenomenon “asterism” is most visible in direct, single pointed light source, like an incandescent bulb, fibre optic light, and penlight or in sunlight.
A NEW METHOD FOR IMITATING ASTERISM
Several gems were recently examined that showed stars with an unnatural appearance or an unusual number of rays. Asterism in some of these gem materials is very rare, or has not been seen previously by the authors. Microscopic examination revealed that these “stars” were produced by using what appeared to be a rough polish to scratch lines in an oriented fashion onto the upper surface of the cabochons.
According to McClure and Koivula, the new type of synthetic asterism is created by orientated series of scratches on the surfaces of the precious gems, cut as cabochon, in which the scratches run parallel to each other. Although details of the treatment method are as yet unknown, it is assumed that the scratches are applied to the surface of the cabochon by hand, whereby the number of series of parallel scratches defines the number of arms in the synthetic stars.
Although it has been known since the 19th century that asterism can be created in metal plates, for example, through oriented scratches, this method had not yet been applied to natural precious gems in order to create synthetic asterism. A detailed treatment of the topic of asterism on even metal sheets is contained in a study penned by W Maier titled Experimental Asterism in the New Journal for Mineralogy, Geology and Paleontology, Vol.78, part 3, 1943, pp 283-380. This article also describes two other examples of synthetic asterism in natural precious gems.
The author purchased two cabochons from a dealer. The seller claimed that the stones originated from India or Sri Lanka. The first cabochon with synthetic asterism that was examined was a reddish brown cabochon of 3.08 ct. Absorption spectroscopy revealed a spectrum with a series of iron bands, which are typical for the members of the mixed crystal family pyrope/almandine garnet. The microscopic examination of the transparent, actually very pure stone reveals only a few rutile needles, running at a slant to the surface of the cabochon.
The orientation and in particular the low number of these rutile needles meant that they most certainly did not contribute to the asterism observed in this stone. The star itself is made up of nine (9) sharp lines of light, caused by nine series of parallel scratches on the surface of the garnet cabochon. In general, four-arm asterism and more rarely six-arm asterism have been described as occurring naturally in various deposits of garnet. A star with nine arms is incompatible with the cubic symmetry of garnet.
The second ‘star’ precious gem examined was an opaque, black tourmaline of 15.04 ct. Its relatively high refractive indices of 1.625 and 1.646 revealed the black tourmaline must be a stone with relatively high ferrous iron content. The asterism observed in this stone consisted of a six- armed star. The pattern and symmetry of the six arms of the star was homogenous with the trigonal symmetry of tourmaline. However, one additional ‘satellite could be observed on one of these arms.
It ran from the center of the cabochon alongside the main arm to approximately the middle of the cabochon. A second, less distinct, additional line also ran six main lines. Parallel stripes and scratches could also be observed on the surface of the cabochon; and these were responsible for the synthetic asterism. At the moment-that is in the stones observed in retail trade up until now, the determination and identification of this kind of manipulation or treatment method to create synthetic asterism has been relatively simple.
A certain number of light lines and/or a symmetry in the star that does not conform to the symmetry of the precious stone itself on which the observed asterism was created synthetically, speaks clearly for manipulation and not for natural asterism. The existence of incomplete arms and light lines with ‘false’ orientation, also known as ‘satellites’ of main arms of the star, also denote that the star was created artificially. The same applies to the parallel scratches that can be observed on the surfaces of the manipulated cabochon under the microscope, if the stones do not have in their centers any ‘needles’ with an orientation and concentration required for the natural creation of a star.
