What does "tone" refer to in the context of gemstones?

In the context of gemstones, "tone" specifically denotes the degree of darkness or lightness of a color. It helps in understanding how vivid or muted a gemstone's color appears. The tone can range from very light to dark, and it plays a significant role in the overall appearance and grade of a colored gemstone. For example, a deeper tone may enhance the richness of a gemstone's color, while a lighter tone may impart a different vibrant quality. Thus, tone is an essential attribute in assessing the beauty and value of colored stones, as it directly affects how their color is perceived and appreciated. The other options relate to different characteristics of gemstones that do not pertain to the direct measurement of lightness or darkness of color.