<p>Biogenic guanine crystals found in various aquatic and terrestrial animals, show unique optical properties that include structural coloration as well as reflectance and have intrigued researchers for over a century. Besides the highly reflective nature of these organic crystals, guanine-rich parts of certain organisms (fishes, geckos, etc.) show emission in the visible region upon UV light exposure. This fluorescence is used by these organisms for intra-specific signaling. However, there is no available literature regarding the occurrence and origin of visible light emission in guanine crystals. Earlier reports suggest that guanine, when excited at its absorption maxima (254 nm), emit in the UV region (λ_em = 332 nm). The objective of the present study is to address the fundamental question: How can guanine crystals emit in the visible region. Laboratory crystallization of guanine requires extremely acidic or basic medium due to its limited solubility under ambient aqueous conditions. Herein, we report blue emissive guanine crystals obtained under highly acidic or basic conditions and explore the origin of their fluorescence in the visible region. Guanine solutions and crystals showed charge-transfer UV–vis bands that, upon excitation, produced blue emission. A combination of experimental and theoretical tools was employed to establish the fluorescence mechanism.</p><p></p>