Asterism Gemstones: Star Gems, Causes, Types

Asterism, also known as the "star effect," is a rare and captivating phenomenon seen in certain gemstones. It appears as a star-shaped light pattern that shimmers and dances as the stone is moved under a light source. This mesmerizing effect is caused by the presence of microscopic needle-like inclusions within the gemstone that reflect light in specific directions.

An asterism is an optical phenomenon displayed by some rubies, sapphires, and other gems (i.e. star garnet, star diopside, star spinel, etc.) of an enhanced reflective area in the shape of a "star" on the surface of a cabochon cut from the stone. 

Asterism Gemstones: Star Gems Types
Asterism Gemstones: Star Gems Types

What causes an asterism?

Asterism in gemstones is caused by a phenomenon known as "silk," which refers to the presence of needle-like inclusions within the crystal structure of the gemstone. These inclusions are typically composed of minerals such as rutile (titanium dioxide) or hematite (iron oxide). The interaction of light with these needle-like inclusions results in the appearance of a star-shaped pattern on the surface of the gemstone. The key factors that contribute to the formation of asterism are:

Orientation of Inclusions: The needle-like inclusions causing asterism must be oriented in a specific way within the gemstone. For a six-rayed star (the most common type), the needles need to be arranged in three pairs along three directions that intersect at 60-degree angles, coinciding with the crystal's three-fold symmetry axis.

Light Reflection: The inclusions scatter and reflect light in such a way that a star-shaped pattern becomes visible when the gemstone is illuminated. The reflection occurs because the needles interrupt the passage of light and create reflective surfaces.

Cabochon Cut: Gemstones that exhibit asterism are often cut into cabochon shapes. A cabochon is a gemstone that has been shaped and polished into a rounded, domed form without facets. This cut enhances the visibility of the star effect by allowing light to interact with the inclusions across a smooth, curved surface.

Gemstones that display asterism may exhibit four, six, and sometimes twelve rayed "stars", depending on the inclusions, size, and facet.

A distinction can be made between two types of asterism:

  • Epiasterism, such as that seen in sapphire and most other gems, is the result of a reflection of light on parallel arranged inclusions inside the gemstone.
  • Diasterism, such as that seen in rose quartz, is the result of light transmitted through the stone. In order to see this effect, the stone must be illuminated from behind.
Asterism Gemstones
How Do Asterism Minerals Form?  Asterism on the surface of a star sapphire

Which gemstones exhibit asterism?

Star Sapphire: Perhaps the most well-known gemstone with asterism is the star sapphire. This variety of corundum (aluminum oxide) often contains needle-like inclusions of a mineral called rutile. When these inclusions are aligned in a way that intersects with the crystal's three-fold symmetry axis, a six-rayed star appears on the surface of the gemstone.

Star Ruby: Like star sapphires, star rubies are also a type of corundum that exhibits asterism. The red color in star rubies is due to the presence of chromium. When the ruby contains aligned needle inclusions, it can display a six-rayed star when exposed to light.

Star Garnet: Some varieties of garnet, particularly almandine and rhodolite, can exhibit asterism. The star garnet typically displays a six-rayed or four-rayed star and is most commonly found in red or purplish-red hues. To date, they have only been found in two locations worldwide: India and the Idaho Panhandle in the United States.

Star Diopside: This unique gemstone showcases a four-rayed star in a deep black or dark green color. It's known for its striking contrast between the dark body and the bright star. The asterism in star diopside is often caused by needle-like inclusions of magnetite or other minerals.

Star Rutilated Quartz: This gemstone features a six-rayed star caused by rutile needles within the quartz. It can come in various colors, including clear, yellow, green, and brown.

Star Moonstone: Asterism is observed in some varieties of moonstone, particularly those with a milky or bluish adularescence. The phenomenon in moonstone is due to the presence of thin, parallel-oriented fibers or needle-like inclusions of minerals such as rutile or goethite.

Star Rose Quartz: Rose quartz, a variety of quartz with a pink to rosy color, can sometimes exhibit asterism. The star effect in rose quartz is usually less pronounced than in other gemstones, and the star may have fewer rays.

Star Spinel: Star spinel is another gemstone that can exhibit asterism. However, star spinels can come in a wider variety of colors than star garnets, including red, pink, orange, purple, blue, and green. Star spinels may display a 4 or 6-rayed star depending on the cutting and viewing angle.

These gemstones with asterism are often cut into cabochons (domed, polished shapes without facets) to best display the star effect. The value of a star gemstone is influenced by the intensity and visibility of the star, as well as the overall quality of the gem.

See also:
Types of opal with photos
Types of Amber with PHOTOS
What is the difference between minerals and crystals?
Purple Gemstones and Crystals
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