Date of Award

2026-08-01

Degree Name

Doctor of Philosophy

Department

Chemistry

Advisor(s)

Jorge L. Gardea-Torresdey

Abstract

Corundum-based red gemstones, commonly known as rubies, are among the most valuable-colored gemstones. However, their accurate identification is complicated by synthetic production, post-growth treatments, and the availability of visually similar materials such as red spinel, garnet, tourmaline, as well as glass imitations. The study established that conventional gemological measurements provide useful preliminary evidence for the characterization of rubies, but the combination of spectroscopic measurements can reliably establish their identity, origin, and treatment history. This dissertation develops and evaluates a nondestructive, multimethod framework that integrates conventional gemological measurements with X-ray fluorescence (XRF) spectroscopy, Raman spectroscopy, and hyperspectral microscopy (HSM) to characterize corundum-based rubies and distinguish them from other red gemstones, to investigate natural and synthetic origins, as well as to screen for potentially toxic elements' presence in the red gemstones. Chapter 1 represents a critical review of recent advances in hyperspectral microscopy and its applications across biomedical science, pathology, environmental forensics, ecotoxicology, food-quality assessment, materials science, nanotechnology, horticulture, and planetary research. The review identifies the technique's principal advantages, including high spatial resolution, label-free analysis, and the ability to detect subtle spectral variations. This review establishes the scientific basis for applying HSM to gemstone characterization. Chapter 2 evaluates a collection of 122 gemstones comprising one certified natural ruby, eight certified synthetic rubies, and 113 commercially obtained red gemstones. The samples were examined using refractive index, specific gravity, thermal conductivity, optical microscopy, high- and low-wavenumber Raman spectroscopy, HSM, and XRF. Conventional measurements provided effective preliminary screening where 59 of the 113 commercial samples exhibited birefringent refractive-index values of approximately 1.762 - 1.771, while specific-gravity values near 4.0 and relatively high thermal conductivity further supported a corundum classification. Rubies consistently exhibited high-wavenumber features near 4391 and 4361 cm-1 and low-wavenumber lattice modes near 209 and 171 cm-1, supporting the presence of crystalline a-Al2O3. Garnets were differentiated by a prominent band in the approximately 914 - 921 cm-1 region, whereas spinel and glass imitations generally lacked the characteristic corundum signatures under the analytical conditions employed. Hyperspectral analysis revealed that 48 samples (42.5%) displayed a feature near 690 nm alone, while 15 samples (13.3%) exhibited features near both 690 and 475 nm. Fifty samples (44.2%) showed neither feature nor no sample displayed the 475 nm feature alone. The dominant feature near 690 nm was attributed to Cr3+ electronic transitions within the corundum crystal field. The combined occurrence of the 690 and 475 nm features, interpreted additional evidence for differentiating natural and synthetic materials. XRF analysis complemented these techniques by identifying chromium and other trace elements associated with coloration, geological formation, synthesis, or treatment. Although the instrument could not detect aluminum under the conditions used, the trace-element profiles helped distinguish Cr-bearing corundum from Fe-, Zn-, Mn-, and Pb-containing alternatives or treated materials. Integration of all measurements classified approximately 63% of the 113 commercial samples as rubies, compared with approximately 12% garnets, 7% spinels, and 1% red tourmaline, with the remaining samples representing glass imitations. Fifteen samples were identified as probable natural rubies, while the remaining ruby-classified samples were categorized as synthetic. Chapter 3 extends the XRF investigation to screen the gemstones for presence of potentially toxic elements (PTEs). In this study, lead (Pb) was detected in 13 samples, arsenic (As) in 10 samples, cadmium (Cd) in one sample, as well as nickel (Ni) was found in 12 samples. The co-occurrence of Pb and As in several samples suggested possible associations with glass filling, pigmentation, coatings, composite construction, or other treatments indicating probable exposure risk to human health.

Language

en

Provenance

Received from ProQuest

File Size

136 p.

File Format

application/pdf

Rights Holder

Kazi Saima Banu

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