PL spectroscopy has also proved useful for somecolored stone applications. For example, separatingnatural from synthetic spinel can be difficult in high-clarity gems. However, PL analysis of samples withtrace or higher concentrations of chromium can easi-ly distinguish laboratory-grown material from natu-ral spinel (figure 7; Notari and Grobon, 2003; Shen etal., 2004; Kitawaki and Okano, 2006). Similar fea-tures provide evidence for the heat treatment of nat-ural spinel to enhance color (Saeseaw et al., 2009).Additionally, PL analysis can be combined withRaman spectroscopy to separate natural red coralfrom its dyed counterpart