We propose a new method of synthesis of crystalline yttrium and lutetium orthoborates in thin (2.5–20.0 μm thick) layers of molten borate oxide glasses. The method is based on interaction of nanosized lutetium and yttrium oxide powders with boron anhydride in the melt. X-ray diffractometry and electron microscopy investigations have revealed that in the case of lead-content glasses, the method shows a very strong orienting effect in the texture of the resulting orthoborate particles. This highly oriented texture results in good transparency of the obtained glass–ceramic films. Studies of the luminescence and scintillation spectra of the glass–ceramic composites show that they can be used as highly effective phosphors with a light yield comparable to that of single crystalline scintillators.