This article reviews blue shortwave-excited luminescence (BSL) in natural minerals and synthetic materials. It also describes in detail the emission of seven minerals and gems displaying BSL, as well as three references in which BSL is caused by titanate groups (TiO6): benitoite, Ti-doped synthetic sapphire and spinel. Emission (under 254 nm shortwave excitation) and excitation spectra are provided, and fluorescence decay times are measured. It is proposed that BSL in beryl (morganite), dumortierite, hydrozincite, pezzotaite, tourmaline (elbaite), some silicates glasses, and synthetic opals is due to titanate groups present at a concentration of 20 ppmw Ti or above. They all share a broad emission with a maximum between 420 and 480 nm (2.95 to 2.58 eV) (thus perceived as blue), and an excitation spectrum peaking in the short-wave range, between 230 and 290 nm (5.39 to 4.27 eV). Furthermore, their luminescence decay time is about 20 microseconds (from 2 to 40). These three parameters are consistent with a titanate emission, and to our knowledge, no other activator.

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There are three main colors of luminescence in corundum (Al 2 O 3 ). First, the well-known red luminescence of Cr 3+ , second, the blue luminescence related to titanium and, third, the orange one which is the subject of this work. There are few discussions of the origin of this orange emission in corundum, except that it is believed to be related to a hole center. We characterized about 50 samples of mostly natural corundum from five different localities, synthetics and Be-diffused gems, using classical gemology, LA-ICP-MS, EPR and luminescence spectroscopies.Chemical analysis revealed relatively pure corundum, with a low concentration of iron and high concentration of divalent cation (mostly Mg 2+ ) compared to other, non-luminescent corundum. The Fe vs Ga/Mg ratio indicates always a metamorphic origin.The orange luminescence is associated with a broad emission band (about 200 nm FWHM) centered between 610 and 630 nm. The luminescence decay time is about 1-10 µs. Heating at about 1273 K does not influence the emission or excitation. The excitation spectrum associated with this luminescence revealed always the same absorptions, in different relative proportions, located at about 253, 315, 353, 415, 485 and 563 nm.. Physics literature associates these absorption/excitation bands with a family of defects called the F-centers (i.e. oxygen vacancies for corundum). These defects are known to be common in many oxides. These centers may accommodate the charge imbalance due to the presence of divalent cations, as detected in LA-ICP-MS. Unfortunately, in ESR, due to the everpresent signal of Cr 3+ , F-centers cannot be visible. Nevertheless, all our results point consistently to attribute orange emission to F 2 -centers related to divalent cations that is two oxygen vacancies coupled most likely to two Mg 2+ . Most of the excitation is caused by F-centers as well, but occasionally excitation energy is transmitted through Cr 3+ , V 3+ and even rarely Mn 4+ .