Four millennia of garnet trade in Northeast Africa—chemical analysis of ancient and Late Antiquity beads from Lower Nubian sites
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2024-03-07 |
| Journal | Archaeometry |
| Authors | H. Albert Gilg, Joanna Then‐Obłuska, Laure Dussubieux |
| Institutions | Field Museum of Natural History, University of Warsaw |
| Citations | 4 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This study utilized advanced analytical techniques to establish the provenance and chemical fingerprint of red garnet beads spanning four millennia (3200 BCE-600 CE) in Northeast Africa.
- New Material Identification: A new, chemically distinct garnet type, designated “Cluster I,” was identified. This type is a calcium-poor almandine (69%-78% almandine, 15%-22% pyrope).
- Local Sourcing Confirmed: Cluster I garnets were definitively sourced to alluvial/eluvial deposits in Upper Nubia (Bayuda desert, Wadi Abu Dom and Wadi El Haraz), approximately 670 km from the Lower Nubian grave sites.
- Unique Geochemical Signature: Cluster I is characterized by high Yttrium (Y: 180 to 1205 ppm), high Scandium (Sc: 119-213 ppm), and moderately low Chromium (Cr: 13-70 ppm), distinguishing it from known Indian and European garnet types (Clusters A, B, G).
- Technological Advancement: The study confirms that Northeast Africa is the cradle for the oldest use of almandine garnet, a gemstone harder than quartz (Mohs 7.5), requiring specialized abrasive techniques for manufacture.
- Trade Network Evidence: The consistent use of the Nubian Cluster I garnet for 3,500 years demonstrates a robust, long-term regional trade network between Upper and Lower Nubia, and potentially Egypt.
- Imported Material Identified: A single, younger Post-Meroitic bead (ISAC 34) was identified as a Ca- and Mn-poor pyrope (52% pyrope), featuring diamond-tipped drilling and specific inclusions, strongly suggesting importation from a South Asian site (e.g., Sri Lanka or India).
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Garnet Type (Dominant) | Cluster I (Almandine-Pyrope) | N/A | Found in A-Group to Meroitic beads |
| Almandine Content (Cluster I) | 69-78 | % | Major component of Nubian beads |
| Pyrope Content (Cluster I) | 15-22 | % | Minor component of Nubian beads |
| Grossular Content (Cluster I) | 2-6 | % | Minor component of Nubian beads |
| Spessartine Content (Cluster I) | 3-9 | % | Minor component of Nubian beads |
| Yttrium (Y) Concentration | 180-1205 | ppm | Defining trace element range for Cluster I |
| Scandium (Sc) Concentration | 119-213 | ppm | Defining trace element range for Cluster I |
| Chromium (Cr) Concentration | 13-70 | ppm | Defining trace element range for Cluster I |
| Post-Meroitic Bead (ISAC 34) | 52% Pyrope, 45% Almandine | Mole % | Imported, Ca- and Mn-poor composition |
| Raman Band V (F2g mode) | 631 to 635 | cm-1 | Characteristic of almandine-rich Cluster I |
| Raman Band IV (Low Ca) | 861 to 867 | cm-1 | Characteristic of low Ca content in Cluster I |
| LA-ICP-MS Beam Diameter | 70 | µm | Microinvasive spot size for analysis |
| LA-ICP-MS Laser Energy | 0.6 | mJ | Operating energy (70% of maximum) |
| LA-ICP-MS Pulse Frequency | 20 | Hz | Ablation rate |
| Mohs Hardness (Almandine) | 7.5 | N/A | Requires abrasive harder than quartz (7) |
Key Methodologies
Section titled “Key Methodologies”The study relied on two primary analytical techniques for non-destructive and microinvasive characterization:
1. Raman Spectroscopy (WITec alpha300 R)
Section titled “1. Raman Spectroscopy (WITec alpha300 R)”- Purpose: Non-destructive identification of major elemental composition based on Raman band positions (related to SiO4 bending and rotation modes).
- Excitation Source: Nd:YAG laser operating at 532 nm.
- Laser Power: 20 mW.
- Grating: 1800 grooves/mm.
- Acquisition Parameters: Three accumulations, 10 s integration time each.
- Key Result: Confirmed almandine-rich composition for Cluster I (low Raman shifts for bands V and XII) and pyrope-rich composition for ISAC 34.
2. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS)
Section titled “2. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS)”- Purpose: Quantification of major, minor, and trace element compositions for chemical fingerprinting and sourcing.
- Instrumentation: Thermo ICAP Q ICP-MS coupled with Elemental Scientific Lasers NWR 213 system.
- Ablation Parameters:
- Mode: Single point analysis.
- Beam Diameter: 70 µm.
- Laser Energy: 0.6 mJ (70% power).
- Pulse Frequency: 20 Hz.
- Pre-ablation Time: 20 s (to eliminate surface contamination).
- Gas Flow: Helium used as carrier gas (laser), Argon used in the plasma.
- Calibration and Correction:
- Internal Standard: 29Si.
- External Standards: NIST SRM 610, Corning Glass B and D.
- Correction Applied: Major element concentrations corrected for a 13% systematic relative deviation observed in Calcium (Ca) measurements compared to electron microprobe data.
Commercial Applications
Section titled “Commercial Applications”The findings and methodologies have direct relevance to modern materials science, manufacturing, and supply chain integrity:
- Advanced Abrasive Manufacturing:
- Garnet’s high hardness (Mohs 7.5) makes it a superior abrasive. The characterized almandine composition (Cluster I) is relevant for optimizing waterjet cutting media, sandblasting, and high-precision lapping/polishing compounds.
- Gemstone and Mineral Sourcing Technology:
- The established trace element fingerprints (high Y, Sc, low Cr) provide a robust geochemical model for verifying the provenance of modern garnet supply chains, crucial for ethical sourcing and anti-counterfeiting in the jewelry industry.
- Solid-State Laser Host Materials:
- The high Yttrium concentrations (up to 1205 ppm) found in the natural Cluster I garnets are highly relevant to the synthesis of Yttrium Aluminum Garnet (YAG) crystals, which are fundamental host materials for high-power solid-state lasers (e.g., Nd:YAG).
- Micro-Analysis and Quality Control (QC):
- LA-ICP-MS: The microinvasive technique used (70 µm spot size) is directly applicable to QC in semiconductor manufacturing and thin-film analysis, allowing for precise trace element mapping and impurity detection in complex material stacks.
- Raman Spectroscopy: Used for rapid, non-destructive material verification, phase identification, and stress analysis in industrial materials, including ceramics, polymers, and synthetic gemstones.
View Original Abstract
Abstract Raman spectroscopy and laser ablation‐inductively coupled plasma‐mass spectrometry were used to characterize the chemical composition of 34 red garnet beads from Lower Nubian sites, dated between about 3200 BCE and 600 CE. All beads from the A‐Group to the Meroitic period feature a similar calcium‐poor almandine composition (69%-78% almandine, 15%-22% pyrope, 2%-6% grossular, 3%-9% spessartine), which differs from other calcium‐poor almandine garnet types, sourced mostly from Indian deposits in Antiquity. The Nubian beads constitute a new garnet type, named “cluster I”, featuring high yttrium (180 to 1205 ppm), moderately low chromium (13-70 ppm), and high scandium (119-213 ppm) concentrations. Their compositions match with previous and two new analyses from two alluvial garnet deposits, Wadi El‐Haraz and Wadi Abu Dom, near the Fourth Cataract of the Nile in Upper Nubia, about 670 km as the crow flies from the Lower Nubian graves. Garnet trade between the Bayuda desert and Lower Nubia sites, and possibly even Egypt, flourished for almost four millennia. Northeastern Africa is the cradle for the oldest use of a gemstone that is harder than quartz—the red almandine garnet. A Post‐Meroitic bead, the youngest in the assembly, displays an unusual faceting, a diamond tipped drill hole, excellent polish, distinct short‐ and long‐prismatic colorless mineral inclusions, and a calcium‐ and manganese‐poor pyrope composition. This suggests that it was not of a local, Nubian, production, but imported, most probably from a South Asian site.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
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