(Gem of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

Go to: Homepage -- what's new in faceted gems -- what's new in designer cabochons and gem carvings -- gem of the month -- gem of the month archive -- birthstone of the month -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- about the ACS cutters -- settings for these gems --faceting information -- purchase UltraTec equipment

 

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April, 2002 (Revised: October, 2004)

Star Stones (Asterism)

When parallel, needle-like inclusions, or 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 star stones. The most commonly seen examples are star corundums; 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 nearly invariably done to sapphire rough, dissolves rutile needles, clarifying and sometimes color enhancing the stone, yes, -- but eliminating potential stars!

[Rutile needles aligned in three directions in corundum]

[Bicolored star sapphire / white star sapphire / star ruby]

 

The only other gem which commonly forms stars is quartz, where the phenomenon tends to be more visible in transmitted than reflected light, in this species, rose quartz is the most frequently asterated variety. Most citrine in commerce has been heated which tends to dissolve the fine rutile inclusions necessary for star formation, so it is rather rare. In extremely fine, near transparent stones which have been cut to near spherical shape, multiple stars form an intersecting pattern over the surface.

[Star rose quartz / Star citrine]

[Multi-star quartz]

Synthetic star corundum has been produced for several decades in a process whereby a high concentration of very short rutile fibers are added to the crucible as the raw materials are melted, and the resulting crystals are cooled in a very controlled manner. Such "Linde" sytle stars look extremely uniform and bright --- they almost seem painted on the surface rather than to emanate from within the stone. In more recent years diffusion processes have been developed by which natural or synthetic stones can be star enhanced. ACS offers only fully natural star stones.

[Synthetic star ruby]

Other much less commonly found star stones include diopside, enstatite, moonstone and garnet which show four rayed stars and beryl and spinel which usually show six rayed stars.

[Star diopside / star moonstone / star spinel]

 

In some cases natural patterning, twinning growth zones or inclusions can form a four or six sided figure in the a stone, but as these are not dependent on light and are a permanent part of the gem, they are not considered star stones.

Not Star Stones!

[Rutile-hematite starburst in quartz / inclusion stained channels in chalcedony / chiastolite: Andalusite crystal with carbon inclusions]

Star stones are usually native cut and often have a bulging belly and an uneven bottom which can complicate the setting process for jewelers. Rather than this indicating lack of skill or care, we should consider that native cutters are very skilled at maximizing the star potential of any piece of rough. Depth in the bottom of the stone can increase the clarity of the star and controlled unevenness can often help center the star in rough that is difficult to orient.

Speaking in terms of corundum and quartz, star stones have good wearability and are often chosen for rings. When choosing a star stone for your collection or for jewelry wear, it is important to remember that the star you see is a function of the quantity and quality of available light; and it will show itself to best advantage only in strong light, such as sunlight or a single overhead light source indoors. Under dim, diffuse or multiple light sources all but the strongest star stones will merely look chatoyant.


Value Factors

Several factors influence the value of star stones. First would be the rarity of the material itself. For example, star beryl or star spinel have inherent rarity value not possessed by star quartz or corundum, since these species so rarely form stars. Secondly, within any gem category value rises with the same three basic parameters that control most of any gem's value: color, clarity and carat weight.

In general, the more saturated the color, the more translucent, the fewer distracting, visible inclusions, and the larger the size, the more valuable a star stone is. Added to these basics are the characteristics of the star itself. The best pieces have strong stars which show themselves in less than ideal lighting conditions. These stars have straight, evenly bright legs which reach all the way to the girdle of the stone and are nicely centered in the gem.

Gemological Properties: (these values are for sapphire -- will vary with gem species)

Makeup: Aluminum oxide

Hardness: 9

Toughness: Good

Crystal System: Trigonal

Luster: Vitreous to silky

Density: 4.00

RI: 1.76-1.77

Birefringence: .008


Stones Currently Available:

{Search our Catalog} search on "star" to see all species, or star, species (like ruby, etc) to see just one type

Go to: Homepage -- what's new in faceted gems -- what's new in designer cabochons and gem carvings -- gem of the month -- gem of the month archive -- birthstone of the month -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- about the ACS cutters -- settings for these gems --faceting information -- purchase UltraTec equipment

 

{Search our Catalog}