Star sapphire gemstone with asterism effect.
Radiant star sapphire shimmering in the light.

Corundum Star Gemstones: Information Zone

Corundum Star Gemstones must be cut as cabochons in order to exhibit asterism. The cause of the asterism is exsolved rutile or hematite ‘silk’, multitudes of tiny oriented needles or plates. These inclusions form by exsolution – the unmixing of a solid solution. At high temperatures, the corundum may absorb more impurities, but as the crystal cools, the impurities can sometimes be forced out of solution. Since they are still in a solid, the impurity atoms only crystallize where there is space and this is determined by the host structure.

In corundum, exsolved rutile and hematite crystallize in three directions in the basal plane, parallel to the faces of the hexagonal prism. When a stone contains enough of such inclusions, it can display a six rayed star effect if it is cut as a cabochon, with the base of the cab parallel to the basal pinacoid (parallel to the layers of silk). The hematite crystallizes parallel to the first-order hexagonal prism while the rutile follows the second-order hexagonal prism (30 off the first-order prism). If a stone contains enough of both hematite and rutile silk, it can display a twelve rayed star. This is fairly common in black star sapphires, but is rare in other colors. To observe the star effect the stone should be examined in direct sunlight, or under a single, intense light, such as a fiber-optic light, pen light or spot light.

Unveiling the Mystique: Exploring Corundum Star Gemstones

Corundum, renowned for its captivating hues and remarkable durability, holds a special allure among gem enthusiasts. Among its various forms, corundum star gemstones stand out as mesmerizing marvels, captivating admirers with their celestial-like phenomena. Delving into the realm of corundum star gemstones unveils a fascinating narrative of rarity, beauty, and mystique.

Understanding Corundum Star Gemstones:

Corundum stars, also known as asteriated corundum, are a captivating variety of this gemstone renowned for their unique optical phenomenon known as asterism. Asterism is the phenomenon wherein a gemstone exhibits a star-like pattern when viewed under a direct light source. This mesmerizing effect is caused by the presence of oriented needle-like inclusions within the crystal structure of the corundum.

Formation and Characteristics:

Corundum stars are typically formed through a combination of specific growth conditions and the presence of rutile or hematite inclusions within the crystal lattice. These inclusions, arranged in intersecting planes, interact with light to produce a luminous star pattern typically featuring six rays. The most prized corundum stars exhibit a sharp, well-defined star that appears to float magically atop the gem’s surface.

Varieties and Colors:

Corundum stars are most commonly associated with the colors red and blue, owing to the presence of chromium and iron/titanium impurities, respectively. Rubies with asterism are particularly coveted, showcasing a rich red hue with a striking six-rayed star. Blue sapphires with asterism are also highly sought after, displaying a velvety blue tone accentuated by a luminous star. While these colors are most prevalent, corundum stars can also occur in other hues such as pink, orange, and purple.

Optical Properties and Phenomena:

The mesmerizing star effect displayed by corundum stars is a result of light interacting with the aligned needle-like inclusions within the gemstone. When illuminated with a single, focused light source, such as a pinpoint of sunlight or a spotlight, the inclusions scatter and reflect light, creating the illusion of a radiant star shimmering across the gem’s surface. This optical phenomenon enhances the gem’s allure, adding a celestial touch to its beauty.

Symbolism and Cultural Significance:

Throughout history, corundum star gemstones have held special significance in various cultures and traditions. In many ancient civilizations, these stars were believed to possess mystical powers and were often associated with celestial deities and cosmic energies. Today, corundum stars continue to evoke a sense of wonder and fascination, symbolizing guidance, protection, and the inherent beauty of the universe.

Collectibility and Investment Value:

Due to their rarity and captivating beauty, corundum star gemstones hold significant collectible and investment value. Fine-quality specimens with well-defined stars command premium prices in the gemstone market, attracting discerning collectors and investors alike. Additionally, the enduring popularity of corundum stars ensures their continued demand, making them a desirable addition to any gemstone portfolio.

Care and Maintenance:

While corundum stars boast exceptional durability, care should be taken to preserve their beauty and integrity. Avoid exposing these gems to harsh chemicals, extreme temperatures, or sudden impacts that may damage the delicate inclusions responsible for the star effect. Regular cleaning with mild soap and water, along with gentle handling, will help maintain the gem’s brilliance and luster for generations to come.


Corundum star gemstones epitomize the enchanting beauty and timeless allure of the mineral kingdom. From their celestial-like star patterns to their vibrant hues and enduring symbolism, these captivating gems continue to captivate hearts and minds around the world. As we marvel at the luminous splendor of corundum stars, we are reminded of the profound connection between the earthly and the celestial, encapsulated within these mesmerizing treasures. – Embark on a Celestial Journey with Corundum Star Gemstones.

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