Atomic and microstructural origin of banded colours in purple-blue variety of agate from Yozgat Province, Turkey

Abstract Agates from Yozgat province are appreciated on the gem market for their white and purple-blue banded colours. In this study, we present a detailed investigation aimed at identification of atomic structural origin peculiar colouration chalcedony. X-ray diffraction Raman spectroscopy revealed presence fine grains quartz moganite with preferential accumulation latter in blue bands. Near-infrared diffuse absorption spectra show overtones hydroxyls vibrations 1425, 1900, 2250 nm. visible, broad about 500 nm, as well its behaviour low temperatures, is compatible optical activity iron impurities matrices, such that observed amethysts. Peak intensities shapes very similar collected Accordingly, trace-element composition laser ablation inductively coupled mass spectrometry confirmed two regions have Fe content. The perceived changes band colours indeed originated by differences microstructural arrangement size visualised through scanning electron microscopy. White stripes 5 µm 300 nm size, respectively, resulting an accentuated scattering component Therefore, unique shades typical agates combination iron-related colour centres effect.