Research Note documenting a laboratory-based verification campaign of 851 coloured gemstones purchased from 12+ online marketplaces over an eight-month period (September 2024 to May 2025). The study applies a multi-instrument gemmological protocol (RI, SG, darkfield microscopy, FTIR, UV-Vis-NIR, Raman spectroscopy, EDXRF, gamma spectrometry) to quantify non-conformity rates, undeclared treatments, certificate substitution patterns, and radioactivity screening thresholds encountered in external-market listings. Key findings: overall non-conformity rate of 87% (combined misrepresentation and incomplete delivery), 62% undeclared treatments (including 80% of emeralds and near-100% of topazes), certificate substitution on 100% of encountered GIA and Lotus reports within the sample, and 48 stones (approximately 5.6%) exceeding the 74 Bq/g radioactivity threshold. Ten marketplace archetypes are catalogued taxonomically. Instrumentation: Thermo Scientific Nicolet iS5 FTIR, Magilabs GemmoSphere UV-Vis-NIR, Renishaw Raman, Bruker Tracer 5i EDXRF, Nikon SMZ1270 gemmological microscope, Eickhorst LED refractometer, Ortec DigiBASE gamma spectrometer, Mettler Toledo NewClassic MS balance, plus polariscope, dichroscope, and UV lamps (365 nm and 254 nm). Authors: Pierre Lafrance (GG, GIA), Laboratoire Gem Quebec, Laval, Canada. Nathalie Thomas (GG, GIA), Gem and Mineral Federation of Canada, Kitchener, Canada. Declaration of interest: Laboratoire Gem Quebec is a commercial gemmological laboratory. No commercial relationship exists between the authors and any of the stones, vendors, or platforms analysed. The study was self-funded by Laboratoire Gem Quebec. License: Creative Commons Attribution 4.0 International (CC BY 4.0). Preferred attribution When citing or reusing this work, the authors appreciate the following attribution format: Lafrance, P., & Thomas, N. (2026). Non-Conformity in Online Marketplace Listings of Coloured Stones: Eight Months of Laboratory Verification, September 2024 to May 2025. Laboratoire Gem Quebec. https://doi.org/10.5281/zenodo.19699431. Available at https://www.labgemquebec.com/etude-pierre-couleur The paper is free and open under CC BY 4.0. Notification of reuse is appreciated but not required.