Artificial Asterism

(via Gemmology Queensland, Vol.3, No.2, February 2002/ GZ English language translation) Dr Karl Schmetzer / Martin P Steinbach writes:

A new method for synthetically altering precious gems was discovered only recently. In this case it is a method of synthetically creating asterism in natural materials, cut as a cabochon. Up until now, the creation of synthetic asterism, e.g. by diffusion treatment of sapphires, was only applied to precious gems if these precious gems also produced natural star stones.

A new study by S F McClure and J I Koivula, titled A new method of imitating asterism in Gems & Gemology, Vol. 37, No.2, 2001, pp 124-128, now describes precious gem minerals with synthetic asterism, although the gems themselves (sinhalite, cassiterite and samarskite) do not produce star stones naturally. Synthetic asterism is also described for minerals, which have produced natural star stones such as garnet, chrysoberyl or rutile.

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 the 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.