A distinct six-rayed star appears when light strikes this gemstone's surface.
Tiny, needle-like "silk" inclusions cause the star to appear.
Some rubies exhibit a six-rayed star when light is shined on them, an effect known as "asterism." The star, which appears on the surface of a cabochon-cut ruby, will move when the gem is rocked. The star is not fixed on or below the surface of the stone—as the stone is moved the rays become thicker or doubled, and its character changes in a variety of ways.
The interior of a star ruby contains many "silk"inclusions. Needle-like crystals of rutile (titanium oxide) called "silk" cause the star to appear, and when light strikes these rutile inclusions, which intersect at 60angles to each other, a star appears on the surface of the cabochon. These silk inclusions also cause star rubies to be semi-transparent. The star ruby as shown in the photograph on the next page has a strong red color and strikes a precarious balance between high transparency and the visibility of its star, making it a miraculous gem-quality stone.
Corundum (the mineral name of ruby and sapphire) that is red and highly transparent is polished into faceted ruby. Heavily included stones that are not expected to have an attractive appearance if faceted are generally made into cabochons, which emphasize their color. When a cutter comes across a semi-transparent to translucent stone containing silk inclusions, it is fashioned as a star ruby. Star rubies with a distinct red color are especially valuable and rare.
In Sri Lanka, time-tested methods are employed to heat stones to approximately 1,000C (1,850F). This treatment removes a characteristic bluish color from the rubies. This does not cause the red color to get darker, but when purplish star rubies are heated they often become a more pure red. However, when temperatures exceed 1,300C (2,370F), the silk inclusions that cause the star are altered, causing the star to disappear. Also, any liquid inclusions that are present can rupture during heating and cause the rough to fracture, making it useless as a gemstone. Deciding whether or not to heat a stone requires very specialized knowledge and experience.
Ring, Platinum
Star Ruby 1 pc
3.30 ct
Heated
Diamond 8 pc
US $30,000
Name of mineral : Corundum
Mohs hardness : 9
Refractive index : 1.762 ~ 1.770
Specific gravity : 4.00
siri lanka,Possibly heated
Gem quality
The main source of star ruby and star sapphire is Sri Lanka, and most material has a light body color. Small quantities of high-quality star ruby and star sapphire with distinct body colors are mined in the Mogok region of Myanmar and polished without treatment. On the other hand, large quantities of low-quality Indian star ruby and black star sapphire are also available in Bangkok.
Size (mm) : L 7.6 x W 7.2 x D 4.95
Untreated
US $500
This material is available in relatively high levels of transparency. Comparing violet star sapphires to Indian star rubies helps one understand the considerable effect that transparency has on beauty. Because the cabochon surface of the sapphire in the photograph is relatively flat and wide, a star resembling an open umbrella appears across the whole surface of the stone when a strong light is shined on it. It can be appreciated as a gemstone when viewed from above, but the depth of its base seen from the side is a cause of concern.
From a mineralogical standpoint, this is the exact same corundum as any other star ruby, but it differs in both its appearance and beauty. Star ruby is semi-transparent to translucent, but Indian star ruby is nearly opaque. This is the main reason for its low level of beauty. Because supplies are so abundant, Indian star ruby is an inexpensive gem material. Buyers enticed by the name "ruby" should realize that Indian star rubies are useless as jewelry material.
Size (mm) : L 7.9 x W 6.1 x D 5.38
US $40
Size (mm): L 5.9 x W 5.6 x D 2.77
US $8
JUDGING QUALITY
The key to judging quality is simply to compare a number of star rubies and determine their level of beauty and depth of color. Specifically, there are five factors to consider:
1. Color
2. Quality of Star (Is the star distinct even under weak lighting?)
3. Transparency
4. Shape
5. Positioning of Star (Is the star centered on the cabochon?)
It is important that these five factors are all satisfied in a well-balanced manner. Even with a distinct star, material that has a grayish or brownish body color constitutes low quality. However, as long as the star is even barely visible, material with a fine body color can be used in jewelry.
Black Star Sapphire
Minute, needle-like inclusions of minerals such as hematite (a form of iron) act in a similar fashion as silk inclusions, causing a star to appear. In actuality, the sapphire is yellow, golden, or green, but its various inclusions cause it to become a dark brown or black star sapphire. Diopside, a similar-looking brownish mineral, may also show a 4-rayed star. Both of these materials are extremely inexpensive and sold as gift items. They may be thought of as stones that border between gemstones, which by definition are 'beautiful and rare,' and common minerals.
QUALITY SCALE
STAR RUBIES
Heated
HOW TO CHOOSE
When choosing a star ruby for use in jewelry, select one of gem quality if possible, or at least of jewelry quality, with as pure a red color as possible.
Next, considering the appearance of the star and the stone's overall shape and form, choose a gem that appeals to you. The key is to emphasize the color while keeping in mind that, especially in the case of star ruby, there is no such thing as a perfect stone. Usually, the deeper the red color is, the weaker the star will be. A stone in the 10-carat range with a deep red color and high level of transparency, as well as a distinct star, is something that is rarely encountered.
Take care to avoid buying loose stones with bases that are too deep, since this will cause problems during setting.
The value of a three-carat-size, heat-treated star ruby of jewelry quality is approximately US $5,000 for the stone alone. (2001)
A GEM OF VALUE
Weight : 2.19 ct
Size (mm) : L 7.7 x W 5.1 x D 5.37
Untreated
Jewelry Quality
US $4,000
The 3 quality zones as they appear in the quality scale
Quantity distribution of gems that are polished to 3-carat size
