An optically active particle and a method of manufacturing said particles, plus methods of tagging a material of interest with said particles are disclosed. The particle comprises a surface-enhanced spectroscopy (SES) active core and a SES active reporter associated with the SES core wherein the particle produces a measurable and thermally stable SES spectrum upon optical interrogation. As used in the disclosure, thermally stable may be defined as maintaining a measurable SES spectrum after the particle or any material tagged with the particle has been exposed to a temperature substantially higher than room temperature.