In response to heat and UV exposure, some synthetic diamond gemstones grown by chemical vapor deposition exhibit large, reversible changes in color. A significant reduction in color was achieved by heating several CVD synthetic gemstones to 450°C. Conversely, a darker color was observed in samples following exposure to UV radiation (such as that used in gem testing). Both the heated and UV-exposed samples returned to their initial (stable) color when they were illuminated for >30 minutes with a standard daylight-equivalent lamp used for grading. However, these color states did not change with time when the samples were kept in the dark. Heating and UV exposure also influenced the strengths of various IR absorption features that might be used to identify such a sample as a CVD synthetic. These nonpermanent changes might affect the apparent color grade of a CVD synthetic diamond, and care must be employed in the interpretation of spectroscopic features used to determine a stone’s natural or synthetic origin.