Synthetic star sapphires and rubies, often referred to as “Linde stars” due to their historical association with the Linde division of Union Carbide, represent a fascinating chapter in the history of gemology. These gemstones, produced primarily from the 1960s onward, offered an accessible alternative to their natural counterparts and played a significant role in the development of synthetic gemstone technology. This article will explore the history, production methods, gemological characteristics, and identifying features of these intriguing materials, based on available research.
History and Lore
The production of synthetic corundum, including star varieties, began with Auguste Verneuil’s flame-fusion process in the early 20th century. However, the creation of high-quality, asteriated synthetic sapphires and rubies on a commercial scale was a later development. The Linde division of Union Carbide was a key player in this advancement. From 1966 to 1968, Linde produced star material in red, white, blue, and black, with blue accounting for approximately 80% of the production, followed by red (15%), black (nearly 5%), and a very small percentage of white.
The initial focus was on replicating the appearance of natural star gemstones, but experimentation soon expanded to include the creation of various colors, particularly those associated with birthstones, in the late 1960s. This expansion was driven by market demand and a desire to broaden the appeal of synthetic gemstones. While early production focused on basic colors, later innovations involved techniques to enhance or create asterism through post-growth diffusion.
Geological Formation and Sources
Unlike naturally formed star sapphires and rubies, which originate from metamorphic and igneous geological processes, Linde stars are products of a controlled laboratory environment. The primary method employed by Linde was the Verneuil flame-fusion process, also known as the flame-fusion technique. This process involves melting powdered alumina (aluminum oxide) with trace amounts of other metal oxides, such as titanium oxide, in an oxygen-hydrogen flame. The molten material then drips onto a rotating pedestal, forming a cylindrical boule that is subsequently cut into gemstones.
Later, the company transitioned to the Czochralski technique for crystal growth, allowing for greater control over the process and the potential for higher titanium concentrations. However, excessively high titanium levels resulted in milky, unmarketable stones. The raw materials used by Linde contained titanium, which is crucial for the development of asterism, but other coloring agents were often added via post-growth diffusion to achieve desired hues.
The gemstones were produced in the United States, specifically at the Linde facility, up until 1974. Subsequently, production continued under the Linde trade name, but the origin of these later stones is sometimes uncertain, with some originating from Asian markets. Wiede’s Carbidwerk also produced synthetic star sapphires and rubies, utilizing a patented method developed in the 1950s.
Gemological Properties
Linde star sapphires and rubies exhibit gemological properties consistent with synthetic corundum, with some nuances related to their asterism and production methods.
- Chemical Composition: Primarily aluminum oxide (Al2O3) with trace elements, notably titanium oxide (TiO2) responsible for the star effect.
- Hardness: 9 on the Mohs scale, identical to natural corundum.
- Refractive Index: Consistent with synthetic corundum, typically around 1.762-1.770.
- Specific Gravity: Approximately 3.99-4.00.
- Asterism: The defining characteristic of these gemstones is their six-rayed star, caused by aligned inclusions of rutile needles. The sharpness and intensity of the star can vary depending on the concentration, size, and orientation of these inclusions.
A notable feature of some Linde stars is the presence of a “dual-color double-star” pattern. This manifests as a white star related to the dome of the cabochon and a bodycolored star confined to the base. This phenomenon is a result of the diffusion process used to enhance or create color.
Older “stock” Linde materials often exhibit microscopic features such as gas bubbles, irregular grain boundaries, and Plato lines, indicative of the flame-fusion growth process. The size of the rutile needles can be very fine, sometimes at the resolution limit of a standard gemological microscope.
Symbolism and Metaphysical Beliefs
The sources provided do not contain information regarding the symbolism or metaphysical beliefs associated with Linde star sapphires and rubies. These aspects are typically linked to natural gemstones and their historical and cultural contexts, which are not addressed in the available materials.
Care and Cleaning
Due to their hardness of 9 on the Mohs scale, Linde star sapphires and rubies are relatively durable and resistant to scratching. However, like all gemstones, they require proper care to maintain their brilliance. Cleaning can be performed with warm soapy water and a soft brush. Avoid harsh chemicals, ultrasonic cleaners, and steam cleaners, as these can potentially damage the gemstones or any applied treatments. Given the potential for diffusion treatments, aggressive cleaning or repolishing could alter the appearance of the star.
Identifying Linde Stars
Identifying Linde stars requires careful gemological examination. Key indicators include:
- Presence of a Star: The six-rayed star is the most obvious characteristic.
- Microscopic Features: Gas bubbles, irregular grain boundaries, and Plato lines are common in flame-fusion synthetics.
- Rutile Inclusions: Examination of the rutile needles can provide clues about the growth process.
- Dual-Color Double-Star: The presence of a white star and a bodycolored star is a strong indicator of diffusion treatment.
- Grinding and Repolishing: Grinding and repolishing the base of the cabochon can reveal whether the star is surface-related (diffusion-treated) or integral to the entire stone. A decrease in star intensity after grinding suggests diffusion treatment.
- Inhomogeneous Crusts: Examination of the crusts on partially finished cabochons can reveal details about the production techniques used.
It is important to note that Wiede’s synthetics can sometimes be more difficult to distinguish from natural stones due to the broader, less sharp arms of their stars, which can resemble those found in natural gemstones.
Conclusion
Linde star sapphires and rubies represent a significant achievement in synthetic gemstone technology. Their production, driven by market demand and innovative techniques, provided an accessible alternative to natural star gemstones. While these synthetics lack the geological history and natural formation processes of their counterparts, they possess unique gemological characteristics and identifying features that make them fascinating subjects for study and appreciation. The development of techniques like post-growth diffusion further demonstrates the ingenuity involved in creating these captivating gemstones.