The landscape of gemological science underwent a silent revolution in the 1970s, a period defined by the maturation of classical testing methods and the first widespread emergence of synthetic stones. During this era, the distinction between natural treasures and artificial imitations became the central challenge for museums, auction houses, and private collectors. The discovery of fakes within the National Museum in Prague serves as a stark historical case study: a 19-carat sapphire acquired in the 1970s, once valued in the millions, was revealed to be a synthetic laboratory creation. Similarly, a 5-carat diamond in the museum's collection was identified not as a precious gem, but as cut glass. These revelations highlight a critical gap in the identification protocols of the time. The testing methodologies available in the 1970s relied heavily on optical instruments that were standard practice then and remain foundational today, yet they possessed specific limitations regarding the new wave of hydrothermal and flame fusion syntheses emerging in that decade.
To understand how gemstones were tested in the 1970s, one must examine the specific toolkit of the era. The primary diagnostic instruments were the refractometer and the Chelsea filter, developed in 1934 by Anderson and Payne. These tools were the bedrock of gem identification, designed to measure physical properties that color alone could not reveal. In the 1970s, these methods were applied with great diligence, yet the sophistication of synthetic technology began to outpace the reliability of traditional testing for certain stones. The Prague museum incident, where a sapphire acquired in the 1970s turned out to be an artificial stone, suggests that while the tools existed, the application or the specific diagnostic criteria may have been insufficient to catch the new generation of synthetic materials that were becoming indistinguishable from natural stones using standard optical methods.
The 1970s marked a pivotal moment where the line between natural and synthetic became increasingly blurred. As the National Museum's audit revealed, a stone believed to be a natural sapphire was actually an artificial creation. This stone, purchased for 200,000 crowns in the 1970s, was thought to be worth tens of millions if natural. The error likely stemmed from the limitations of the testing regime at the time. While the tools were present, the specific synthetic materials introduced in the 1970s, such as hydrothermal citrines and flux-grown emeralds, presented new challenges. The testing methods of the era were not "wrong" but were sometimes outmaneuvered by the advancing quality of synthetic production.
The Primacy of Optical Properties Over Color
In the 1970s, gemologists operated under the understanding that color was the most visible trait but the least reliable diagnostic marker. A gemstone's identity was not determined by its hue alone. Instead, experts focused on three distinct color characteristics: hue (color family), tone (lightness or darkness), and saturation (intensity). These attributes, while contributing to the overall identity of a stone, were insufficient on their own. The era was characterized by a reliance on optical properties that provided a more definitive signature for each species.
The fundamental principle of testing in the 1970s was the systematic combination of these observational skills with technological tools. Color assessment was the first step, but it was treated as a preliminary filter. For instance, blue zircons might visually overlap with topaz or spinels, yet their internal optical behavior differed significantly. The 1970s approach required that color be cross-referenced with physical data. This was crucial for antique and vintage jewellery, where stones might have been cut decades or centuries prior, and their identification relied on distinguishing features beyond surface appearance.
The reliance on optical testing was particularly evident in the differentiation of yellow stones. Citrine, for example, was often confused with yellow sapphire, heliodor, or topaz. In the 1970s, the standard procedure involved testing the refractive index to separate these species. However, the emergence of synthetic citrine in the 1970s complicated this process. As noted in gemological records, citrine had been synthesized hydrothermally since that decade. These synthetic versions were increasingly difficult to distinguish from natural stones using traditional methods, creating a vulnerability in the testing chain.
The Refractometer: Measuring the Bending of Light
The refractometer stood as the cornerstone of gem identification in the 1970s. This instrument was used to measure the refractive index (RI) of a gemstone. Each gem species possessed a characteristic RI or a specific range of RIs that acted as a unique fingerprint. The procedure involved placing a small drop of contact fluid on the refractometer's glass surface and then viewing how light bends through the gemstone. The resulting reading would narrow down the stone's identity with high precision.
In the context of the 1970s, the refractometer was applied to a wide variety of stones. For blue gems, such as blue zircons, the refractometer provided a distinct RI that differentiated them from aquamarines or topazes. This tool was particularly valuable for antique jewellery, as testing could sometimes be performed with the stone still in its setting, though the mount could occasionally complicate the reading. The ability to obtain an RI reading was a definitive step in the identification process, providing a hard physical constant that could not be faked by simple visual imitation.
However, the utility of the refractometer had its limits in the 1970s. While it could identify the species of a stone (e.g., distinguishing a sapphire from a glass imitator), it was less effective at distinguishing between a natural stone and a high-quality synthetic of the same species. This was the crux of the issue faced by the National Museum in Prague. A stone could possess the exact RI of a natural sapphire, yet be an artificial creation. The refractometer confirms the material is corundum (sapphire), but it cannot inherently determine origin (natural vs. synthetic) without further analysis of inclusions or specific growth patterns, which required additional tools.
The Chelsea Filter: Dichromatic Insight
Developed in 1934 by Anderson and Payne, the Chelsea filter was a critical diagnostic tool in the 1970s. This optical device transmits only specific deep-red and yellow-green wavelengths. When a gemstone is viewed through this filter, it reveals color reactions that are diagnostic of certain chemical compositions.
The primary application in the 1970s was the identification of chromium-containing emeralds. When viewed through the Chelsea filter, chromium-rich stones would appear red, distinguishing them from green stones that lack chromium content. This was a "go-to" method for differentiating natural emeralds from simulants, a crucial task in an era where emerald imitations were rampant. The filter provided a quick, non-destructive way to screen for the presence of chromium, which is the element responsible for the green color in emeralds and the red fluorescence in rubies.
Despite its historical importance, the Chelsea filter had limitations. It was not a universal test for all stones; it worked best for specific color reactions. In the 1970s, while it was a handy tool for quick checks, it was often insufficient on its own to identify complex synthetic stones that mimicked the chemical composition of natural stones. The filter could confirm the presence of chromium, but if a synthetic stone was also doped with chromium to match the color reaction, the test would return a positive result, potentially misleading the examiner. This limitation is directly relevant to the synthetic sapphire case in the Prague museum. If the synthetic sapphire contained the same chromophores as a natural one, the Chelsea filter might not have provided a definitive "fake" signal, contributing to the initial misidentification.
Microscopic Examination: The Search for Inclusions
In the 1970s, microscopic examination was perhaps the most definitive method for distinguishing natural from synthetic stones. Using a gemmological microscope or a high-quality loupe, experts inspected the internal features of a gemstone. These internal features—known as inclusions, growth patterns, or even tiny cracks—spoke volumes about the stone's origin and treatment history.
For antique and vintage jewellery, understanding these internal traits was essential. In the 1970s, gemologists looked for specific diagnostic inclusions that indicated a natural origin. For example, the presence of "bull's eye" interference figures was a diagnostic marker for quartz. This specific feature, visible under magnification, helped identify citrine and distinguish it from other yellow stones.
The microscopic analysis was also the primary method for detecting treatments. Heat treatment or fracture-filling would leave microscopic "footprints" within the stone. In the context of the 1970s, the ability to see these internal characteristics allowed experts to determine if a stone had been recut, replaced, or if it retained its original condition, which strongly impacted the collectability and value of antique pieces. However, the synthesis of stones in the 1970s began to produce materials with very few inclusions, or inclusions that mimicked natural growth patterns. This made the task of the gemologist more difficult, as the "tell-tale" signs of natural origin were becoming harder to find in high-quality synthetics.
The case of the 1970s synthetic sapphire at the National Museum illustrates this challenge. The stone was likely so well-made that it lacked the typical inclusions that would have flagged it as synthetic under a standard microscope. The inability to find these "natural" signatures, combined with the limitations of the optical tools, led to the false classification of the stone as natural.
The Citrine Challenge: Natural vs. Synthetic in the 1970s
Citrine presented a unique challenge to gemologists in the 1970s. Natural citrine is a variety of quartz, and while it can display a "bull's eye" interference figure, it was often confused with yellow sapphire, heliodor, and topaz. In the 1970s, the market saw the introduction of synthetic citrine produced via hydrothermal synthesis. This method, introduced in that decade, created stones that were visually and optically nearly identical to natural citrine.
The difficulty lay in the fact that many synthetic citrines found on the market were hard to identify using traditional gemmological testing. The standard tools of the 1970s—refractometer, Chelsea filter, and microscope—were insufficient to definitively distinguish these hydrothermal synthetics from natural stones. This technological gap meant that many stones sold as natural in the 1970s were actually laboratory-grown, a fact that might not be discovered until decades later when more advanced spectroscopic equipment became available.
Pleochroism, the variation of color intensity when viewed from different angles, was another factor. Citrine typically exhibits weak pleochroism, showing yellow and pale yellow shades. However, synthetic citrine could also exhibit similar properties. The lack of a clear diagnostic feature to separate the two in the 1970s contributed to the prevalence of misidentified stones in collections. The inability to distinguish the synthetic from the natural became a significant issue for museums and collectors, leading to the re-evaluation of collections years later, as seen in the Prague museum audit.
The Limitations of 1970s Testing and the Case of the Fake Gems
The discovery of fake gems in the National Museum's collection serves as a powerful case study of the limitations of gemological testing in the 1970s. The audit revealed that a 5-carat diamond was actually plain glass with a diamond cut, and a 19-carat sapphire was an artificially created stone. These errors highlight the vulnerability of the era's testing regime.
The glass diamond was a simple imitation, likely identifiable by its low refractive index and lack of thermal conductivity (though thermal conductivity testing was less common in the 1970s, optical properties should have flagged it). The fact that it remained in the collection for decades suggests that the initial identification was based on visual assessment or perhaps a faulty refractometer reading, or the stone was simply too well-cut to be immediately obvious as glass.
The sapphire case was more complex. The stone was acquired in the 1970s, an era when synthetic corundum technology was advancing rapidly. The museum's inability to detect the synthetic nature of the sapphire indicates that the standard tools available then—refractometer, Chelsea filter, and microscope—failed to reveal the stone's artificial origin. This suggests that the synthetic sapphire possessed the same refractive index, color reaction, and internal appearance as a natural stone, effectively fooling the experts of the time.
The difficulty in tracing the origin of these fakes lies in the fact that the responsible personnel from the 1960s and 1970s are no longer available for questioning. The stones were kept under lock and key, and the person in charge of the collection is deceased. This highlights the fragility of historical gemological records and the risk of relying solely on the testing methods of the 1970s. The audit revealed that what was thought to be a natural sapphire worth tens of millions was actually a synthetic stone, a discovery that underscores the sophistication of the synthetic stones of that era.
Synthesis: From Optical Tools to Modern Spectroscopy
The 1970s represented a transitional period in gemology where the tools were evolving to meet the challenges of new synthetic materials. While the refractometer, Chelsea filter, and microscope were the standard, they were often insufficient to catch the highest quality synthetics. The era was defined by a reliance on these "foundational" tools, which provided a holistic picture but lacked the depth of modern spectroscopic equipment.
In the context of antique and vintage jewellery, the identification process required a systematic approach blending observational skills and technological tools. The goal was to form a holistic picture of the stone's identity, ensuring that gems were not misidentified simply because of their surface appearance. The 1970s methodology emphasized that color alone is not definitive, and that physical properties like refractive index and internal inclusions were key.
However, the limitations of this approach became evident when high-quality synthetics were produced that mimicked natural stones perfectly. The case of the National Museum demonstrates that even with these tools, errors occurred. The transition from these traditional methods to more advanced spectroscopic equipment in later decades was necessary to resolve ambiguities that the 1970s tools could not address.
Comparative Analysis of 1970s Testing Methods
To illustrate the capabilities and limitations of the 1970s, the following table summarizes the primary tools and their diagnostic power during that era.
| Tool | Function | 1970s Application | Limitation |
|---|---|---|---|
| Refractometer | Measures Refractive Index (RI) | Identified species (e.g., sapphire vs. glass) | Could not distinguish natural from synthetic if RI matches |
| Chelsea Filter | Filters specific wavelengths | Detected chromium in emeralds (red reaction) | Useless for non-chromium stones or well-doped synthetics |
| Microscope | Inspects inclusions | Identified natural growth patterns (e.g., bull's eye in quartz) | High-quality synthetics lacked distinguishing inclusions |
| Color Assessment | Analyzes hue, tone, saturation | Preliminary screening | Not definitive; many stones share colors |
The table highlights that while these tools were effective for species identification, they struggled with origin determination (natural vs. synthetic) when the synthetic quality was high. This is the exact scenario that led to the misidentification of the Prague museum's sapphire.
The 1970s also saw the introduction of hydrothermal synthesis for citrine. As noted, citrine can be confused with yellow sapphire, heliodor, and topaz. While standard testing could distinguish species, it failed to separate natural from synthetic citrine. This was a critical gap in the testing regime. The synthetic citrines of the 1970s were hard to identify with traditional gemmological testing, leading to potential misclassifications that only modern spectroscopy could resolve.
Conclusion
The 1970s were a complex period in gemstone identification, marked by the maturation of classical optical tools and the simultaneous rise of advanced synthetic technology. The discovery of fake gems in the National Museum in Prague serves as a historical testament to the limitations of these methods. While the refractometer, Chelsea filter, and microscope were the gold standard, they could be bypassed by high-quality synthetics that mimicked the optical properties of natural stones.
The identification of gemstones in this era relied on a systematic approach that prioritized physical properties over color. However, the emergence of hydrothermal citrines and other advanced synthetics created a blind spot in the diagnostic process. The audit of the Prague museum revealed that a 19-carat sapphire, thought to be natural and worth millions, was an artificial creation. This error likely occurred because the stone possessed the same refractive index and color reactions as a natural sapphire, fooling the experts of the 1970s.
Ultimately, the testing methods of the 1970s were robust for species identification but vulnerable to the new wave of synthetic production. The legacy of this era is a reminder that gemstone identification is an evolving science. As technology advanced, the need for spectroscopic analysis grew, but in the 1970s, the reliance on optical tools alone left room for error. The case of the fake gems underscores the importance of continuous methodological evolution to keep pace with the sophistication of gem imitations.