To incorporate different amounts of Co, LLZO single crystal was embedded in LCO powder inside a Cu tube (see Figure ). The tube was then sealed under vacuum and heat treated at different temperatures and dwelling times under Ar to avoid Cu oxidation. After the heat treatment, the embedded single crystals were covered in a solid coating being surrounded by a metallic-appearing surface, which was removed by sanding. After removing the LCO coating single crystals were recovered with colors ranging from almost transparent to blue depending on the treatment history (see Figure ). All single crystals revealed a uniform distribution of the color throughout the whole samples, whereas blue LLZO (LLZO-b) showed also dark blue spots and pale-greenish rim (before polishing.; not visible in Figure ). For further analysis pristine (LLZO-p), orange (LLZO-o) (5 days at 500 °C) and blue LLZO-b (5 days at 900 °C) LLZO single crystals were considered. Such color changes have been observed previously for LLZO mixed with LCO and annealed at temperatures above 600 °C. Moreover, it has been shown that samples tend to become more greenish when higher temperatures are applied. Based on observations made on garnet analogs the color changes could be associated with the incorporation of Co, e.g., Co 2+ on the tetrahedral side (Wykoff position 24d in cubic garnets with space group 𝐼𝑎3𝑑 ). In the Ca3Sb2Fe2Zn1-xCoxO12 (0≤x≤1.0) system the incorporation of Co 2+ on the 24d site causes a change from yellowish, turquoise green to intense green color. Color variations from white, sky blue to dark blue have also been observed in the Ca3Sb2Ga2Zn1-xCoxO12 (0<x≤1.0) system.