| Summary: | Synthetic rock crystals often show a typical infrared (IR) absorption band at 3585 cm<sup>−1</sup>. However, the authors recently found this band in a natural rock crystal with blue coating. The origin of this IR band is controversial as yet. In this paper, the infrared spectra of several natural and synthetic rock crystal samples which were heated to 673 K and 1073 K were measured after these samples returned to room temperature. Comparing the infrared spectra of samples before and after heating, we found the absorption band at 3585 cm<sup>−1</sup> was induced by the thermal process, which indicates that this band cannot be used as diagnostic evidence for synthetic rock crystal alone. In addition, the LiOH bands decreased while AlOH bands increased upon thermal processing. And the negative correlation between the LiOH bands and the 3585 cm<sup>−1</sup> band was also distinct. The above results reveal that the thermal process destroyed the LiOH defects, leading to the formation of a new AlLi defect. And the isolated OH<sup>−</sup> defect inside dislocations generated upon thermal processing is considered to be the exact cause of the 3585 cm<sup>−1</sup> band.
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