“Star Sapphires” The Star on Earth

“Star Sapphires” The Star on Earth

     On a serene night with a clear sky where glittering stars are so vivid, -have you ever dreamt of gathering some stars and keep them in your possession?  Dodging from the abstract imagination to the more concrete and tangible matter, various sparkling gems on earth can be compared to those stars in the sky and they are tactile and realistic. Among the multitude of existing gems, a variety of gems have displayed a 6-ray star effect since the antiquity. Thai people call the gem with this particular phenomenon a “crossed- ray gem” or, in the other word, “star gem”.  In gemology, the display of star on the gem surface is called “Asterism”. Asterism can occur in many varieties of gems, but the best known and the most popular is the beautiful star sapphire. Star sapphire is a variety of the corundum species like ruby, blue sapphire, green sapphire, yellow sapphire which rate the second hardest of gems (next to diamond). Star sapphires exist in various colors and therefore named according to their hue such as: blue star sapphire, black star sapphire golden star sapphire, purple star sapphire, except the red variety is called star ruby.

     Star sapphire is a cabochon cut gemstone (with a dome-like surface and a flat base). Under the incident light, this gem displays a pleasant 6-ray star hovering on the dome surface. This phenomenon is caused by the numerous rutile needle inclusions in the gemstones.  These rutile needles are all parallel in one plane and intersect at 60^o to each other in 6 directions.  It is the intersecting rutile needles inside the stone that reflect the incident light as 6-ray star to our sight.

     Personally, I love star sapphire and it has always been my favorite gem. Since the time I started collecting gemstones, I have never stopped hunting for it. If we sort out a hundred star sapphires of the same size and color and arrange them side by side together, we would see that each single stone is distinct from the other. Each individual stone has its distinct identification: they are unique and are not to others any other: unlike transparent facetted gems of the same cut, size and quality (be it colored or colorless), they seem alike and are difficult to distinguish from each other. Star sapphires remind me of the individuality, like a person with his or her own character and identification that cannot be imitated or simulated. Certainly, this is the appeal of the star gems that I can never resist.

     Gemstones are sold by weight so the gem cutters always work to maintain the weight or, on the other word, to optimize their yield: protecting the weight as much as they can for the maximum yield instead of the good proportion of the stone.  Hence, we usually see that most star sapphires have rather thick body and gem cutters polish only the dome face whereas the back (base) is not polished or even kept as rough condition (hackly or uneven surface). Thereby, this is an easy and practical way to distinguish the natural gems from the synthetic ones. For the synthetic gem, the cutters do not mind the loss of weight so they fashion the gem to the favorable proportion (the ideal) and the back of the gem is well polished. Furthermore, the natural gemstones are often cut according to their original form of the rough pebble; hence they tend to lack the symmetry. This is also another feature and trick to remember when hunting for a natural star sapphire. Interesting to note, star sapphire, in general, are unheated because the height temperature can dissolve the rutile needles inside the stone which could lead to the disappearance of the intrinsic star effect. Hence, the natural star sapphires are normally unheated.

     The main deposits of star corundum gems are in Chantaburi province, especially the area of Bang Kacha, Khao Wua and Khao Ploywaen. Blue star and black star sapphires are the most abundant among other star varieties. Particularly, the black star sapphires which are the mostly exported to the international market. As black star sapphires are very popular in the western countries,  used usually to adorn male jewellery such as : masculine ring , tie clip, necktie pin and cufflinks. The feature of black star sapphire is a solemn dark-background gem that exhibits a contrast to the prominent silvery 6-ray star effect.  Besides the blue and the common black star sapphires, another type of special star corundum is also found in the area of Bang Kacha. It is the star gems that display a distinctly bright yellow or golden star effect. Thai people call it as “yellow star sapphire” or “golden star sapphire”. This golden star sapphire is rather scarce and is found among other blue and common black star sapphires. As miners seldom find it, hence they are considered a rare item. The golden star sapphire is favorite gem among Thai, Japanese and Indonesian. They consider this gem as a lucky charm or amulet, thus they are willing to offer a relatively high price for it.

      For some guidelines, a good star sapphire should have a sharp and complete 6-ray star. The position of star should be well-centered on the dome face (the star that inclines to the side is the result of the cutting that is oblique to the right angle of crystallographic axe). The rays should be straight and extend to the girdle. To check the star, a single lamp is recommended as more lamps may cause confusion. In order to check the right color of the star (the rays), it is important that we use white light (day light) because yellow light (tungsten lamp) may cause a misleading result, particularly in looking for the color of the yellow or golden star sapphire.  Sunlight is ideal for all tests as it allows us to see the true color. It is preferable ,as a guide, to put a common black sapphire side by side with the golden star sapphire to compare and justify the vivid color. The dome face should be smooth and free from fissure or crack that might affect the durability. One should not mind the unpolished, hackly or rough back.  But carefully check the surface with reflected light to verify if it has network-looking or a vein-like appearance with different degree of luster in the corresponding areas. If so, it is possible that such a stone has undergone the fissure or cavity filling treatment:  the foreign material such as high RI glass may be used as filler. Try studying the above techniques in detail before looking for your star sapphires so that you can get the best gem for the appropriate cost and value.

     We are now in June, the beginning of rainy season in Thailand.  Kindly beware of the season change and take care of yourself. Hope to see you in the next article. You can find all previous articles at www.jw.comlab-gem.com

                                                                                      Siriwat Jiamanusorn  (17 June 2016)