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https://pascal-francis.inist.fr/vibad/index.php?action=getRecordDetail&idt=GEODEBRGM71010205 ANISOTROPIC COLOR CENTERS RELATED TO IRON IN AMETHYST QUARTZ. color centers related iron amethyst quartz https://pure.psu.edu/en/publications/in-grown-diamond-color-centers-with-narrow-inhomogeneous-spectral-2/ In-Grown Diamond Color Centers with Narrow Inhomogeneous Spectral Distributions - Penn State diamond color