Carbonado Black Diamond Identification

Understanding Carbonado Black Diamonds: Identification Challenges and Techniques

Carbonado, commonly known as black diamond, presents unique identification challenges distinct from those encountered with traditional, gem-quality diamonds. Its polycrystalline structure and specific geological origin contribute to its characteristics and necessitate specialized testing methods for accurate identification. This article outlines the key features and identification techniques used to differentiate carbonado from other black gemstones and treated diamonds.

Distinguishing Features of Carbonado

Carbonado diamonds possess several key features that, when considered collectively, aid in their identification. These include:

Color: Carbonado is characteristically black, though shades can vary. The color is typically caused by numerous microscopic inclusions, graphite, and amorphous carbon.
Luster: Carbonado exhibits a sub-metallic to adamantine luster, which can appear duller than that of single-crystal diamonds due to its porous structure.
Structure: A defining characteristic is its polycrystalline structure. It consists of numerous microscopic diamond crystals (microcrysts) intergrown together, lacking the single-crystal structure of typical diamonds.
Hardness: Despite its porous structure, carbonado remains exceptionally hard (10 on the Mohs scale), though its toughness may be less than that of single-crystal diamonds due to the grain boundaries between the microcrysts.
Porosity: Carbonado often displays a degree of porosity, which can be observed under magnification. This porosity contributes to its lower density compared to single-crystal diamonds.
Origin: Geologically, carbonado is found almost exclusively in alluvial deposits in Brazil and the Central African Republic. Its extraterrestrial origin is a subject of ongoing research.

Microscopic Examination: A Primary Identification Tool

Microscopic examination is a crucial first step in carbonado identification. At high magnification (50x and above), the polycrystalline nature of the material becomes evident. The intergrowth of microcrysts, along with inclusions and surface features, provides valuable clues.

“Visual examination under magnification is often the first line of defense in identifying potential carbonado specimens. Observing the characteristic polycrystalline texture is key.”

Important features to look for under the microscope include:

Irregular grain boundaries between the microcrysts.
The presence of inclusions, often including graphite, iron oxides, and other minerals.
Surface features indicative of a porous structure.

Density and Specific Gravity Testing

Due to its porous nature, carbonado typically exhibits a lower density than single-crystal diamonds. Standard hydrostatic weighing methods can be employed to determine the specific gravity (SG). While diamond's SG is around 3.52, carbonado's SG usually ranges from 2.9 to 3.4, depending on its porosity and the types of inclusions present. This difference, while not conclusive on its own, is a helpful indicator.

Raman Spectroscopy: Analyzing the Molecular Structure

Raman spectroscopy is a non-destructive technique that provides information about the vibrational modes of molecules and the crystal structure of materials. When applied to carbonado, Raman spectroscopy confirms the presence of diamond's characteristic Raman peak at approximately 1332 cm^-1. It can also reveal the presence of other carbonaceous materials, such as graphite, through additional peaks.

The sharpness and intensity of the diamond Raman peak can vary depending on the quality and crystallinity of the microcrysts within the carbonado. The presence of broad peaks or additional peaks indicative of amorphous carbon or graphite further supports the identification.

X-ray Diffraction (XRD): Confirming Crystalline Structure

X-ray diffraction is a powerful technique used to determine the crystalline structure of materials. When a beam of X-rays is directed at a crystalline sample, the X-rays are diffracted in a pattern that is unique to the material's crystal structure. XRD analysis of carbonado reveals the characteristic diffraction pattern of diamond, confirming its crystalline nature. However, the diffraction pattern may be less sharp or more diffuse compared to that of a single-crystal diamond due to the polycrystalline structure.

Advanced Spectroscopic Techniques: Unveiling Trace Elements and Origin

Advanced spectroscopic techniques, such as Laser-Induced Breakdown Spectroscopy (LIBS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS), can be used to analyze the elemental composition of carbonado. These techniques can identify trace elements that may provide clues about its origin and formation environment. For instance, the presence of certain elements or isotopic ratios may support the hypothesis of an extraterrestrial origin. These techniques are primarily used in research settings.

Distinguishing Carbonado from Treated Black Diamonds

The market includes artificially blackened diamonds, often treated to enhance their color through irradiation or coating. It is crucial to distinguish these from natural carbonado. Key differences and detection methods include:

Treated Diamonds: These often begin as lower-quality, heavily included diamonds that are then treated. Microscopic examination might reveal telltale signs of treatment, such as a concentration of color around fractures or surface irregularities. Color zoning may also be apparent.
Spectroscopic Analysis: Advanced spectroscopic techniques can detect the presence of artificial color centers created by irradiation. Coating can be detected via microscopic examination as well or using techniques like Raman spectroscopy which might be able to identify the coating material.

Careful observation of the diamond's surface and color distribution, combined with spectroscopic analysis, can help differentiate treated black diamonds from natural carbonado.

Avoiding Misidentification: Common Pitfalls

Several common pitfalls can lead to misidentification. Some of these include:

Confusing with other black gemstones: Materials such as black onyx, black spinel, and moissanite can sometimes resemble carbonado superficially. Proper gemological testing, including refractive index measurement and specific gravity determination, can help differentiate these materials.
Relying solely on visual inspection: Visual inspection alone is insufficient for accurate identification. Microscopic examination and other analytical techniques are essential.
Ignoring porosity: The porous nature of carbonado is a key diagnostic feature. Overlooking this aspect can lead to misidentification.

Conclusion: Key Takeaways for Carbonado Identification

Identifying carbonado black diamonds requires a combination of gemological knowledge and advanced analytical techniques. Key takeaways for accurate identification include:

Polycrystalline Structure is Paramount: Microscopic examination to confirm the polycrystalline structure is essential.
Density and Specific Gravity: Measuring density and specific gravity provides valuable supporting evidence. Carbonado typically has a lower density than single crystal diamond.
Spectroscopic Analysis: Raman spectroscopy and XRD confirm the presence of diamond and analyze its crystalline structure.
Distinguish from Treated Diamonds: Careful examination for signs of treatment and the use of spectroscopic techniques are crucial to differentiate carbonado from artificially blackened diamonds.
Expert Consultation: In cases of uncertainty, consulting with a qualified gemologist or gemological laboratory is highly recommended.

By understanding the unique characteristics of carbonado and employing appropriate identification techniques, professionals can accurately assess and value these fascinating gemstones.